Caring for my wife these past 6 years, I have learned a few tactics myself. Everyone's situation and culture will be different (I am a tall man in my mid 30's living in Midwestern US), but let me offer my experience.
During the first few years, I got so good at shutting down these worthless suggestions, that most of my family and close friends don't bother trying any more. Truthfully, it helped, because the rubbernecking and naivete made me feel like a sideshow, or incompetent for not being able to solve something that would apparently be so easy for everyone else. It's possible some people might have gotten their feelings hurt, others might have felt awkward when I opened up to them about what the situation is really like or how I was feeling, but I still think the majority of the time I was polite about it. None of them really avoid us actively, but they don't know what to say... so they don't. Say anything. Everyone just kind of pretends it is all normal, except we don't get invited to things as much anymore, more often hearing about it afterward.
Unfortunately, sometimes I need help, or just want to feel like I am still an important part of someone else's life besides my wife and my son. Sometimes I need someone to babysit during an ill timed doctor's appointment (my son is too young to be left home alone), and sometimes I just want to have a conversation with another adult. I love my wife, but I don't think I need to feel guilty for also wanting to have other friends besides her.
A few years ago, we had to move (we had stairs). In the new town, I decided to take a different approach. It hasn't worked out perfectly yet, but it's better. It has taken some practice, but I focus on two things:
- Reacting to what they mean, not what they say.
- Have a ready list of things I need in my head.
The second is harder, much harder for me (not just because of my machismo, or my experience showing that I can both administrate and execute better than anyone at this point, but because most of what I do and need is reactionary, and can't be planned: hopefully your situation is different), but I have had luck in trying. Never show anyone the full list (it is simultaneously embarrassing and overwhelming), but if you know that you need groceries by Tuesday, someone to sit with her while you shower sometime this week, figuring out a worthwhile task she can accomplish on the good days, a way to get your son to his school thing this Saturday, clean the bathroom, a friend to chat with this next month, a way to get her hair cut, etc., or yes even a casserole, eventually a situation will arise where you can get help with one of those things. I have had many circumstances where I could have asked for help and just couldn't think of anything, again, because most of what I do is reactionary. And I was tired. I don't really have any good suggestions on how to do this, so good luck.
But let me explain the first. Generally, in my experience, no matter what is actually coming out of their mouth, what they are thinking can be categorized in one of 3 ways: a) actually wanting to help, b) gossip fodder, or c) assuage their own guilt. Just like the tabloids, people naturally get curious about other people's lives, I don't see any need to judge someone who feels 'gossipy' at a given moment. And you will talk to many people who are experiencing something akin to survivor's guilt, or feel guilt about not doing more to help you (not enough to actually want to change, just enough to feel bad, which doesn't help anyone). No matter the reason, realizing what they really mean helps me. Whether they are suggesting I get an appointment for her with the 'miracle doctor' that fixed their sister's gout, or asking me how often she falls, it isn't really the words that are coming out of their mouth that are important. People have no idea how to act in these situations, no matter how they are feeling, they just end up making themselves look stupid. It is not always their fault, and eventually you will start getting a chuckle out of their knucklehead behavior. Or get bitter. Sometimes I am bitter. But mostly amused, anymore.
So, here is what I do. No matter their motive, no matter what they actually mean, I look them in the eye, and I always express gratitude. Sometimes, like when I don't really know the person, or have time for a conversation (my son is a normal boy, he will sometimes attempt to grab my hand and physically drag me where he thinks I should go, and he is big enough now that he has some success), all I say is "Thank you." Someone asks "how are you doing" or "how is your wife", "what can I do", "I heard about this root that grows in Antarctica that my neighbor is selling out of his car", the response is still, always, "Thank you." It feels awkward at first, answering questions with a non-answer, and not letting it be cold or dismissive, but actual gratitude. It helps to feel it, grateful that someone actually remembers my wife, is willing to spend even a few seconds of their life to express interest in something that is the most important thing to me... but faking it has worked for me in the past too.
Now, if I actually care about the person (perhaps they know my wife, are family, I think they may want to help, I have time to stop, they are an actual friend, or I think their gossip might get around to someone who actually might care), I do more than just say "Thank you." I will often put my hand on their arm or shoulder, or shake their hand, and look them in the eye, and put actual emotion into those two words. (Anyone who knows me and my wife's condition wouldn't be freaked out by me touching, and it is acceptable in my region.) It is amazing what you can express like that in just a few words: overwhelmed exhaustion, hopeful apprehension, joyful familiarity, whatever it is, it is genuine. Something interesting happens then. It isn't perfect, but at that point, no matter what they actually said, you will more likely be able to see in their face which of the three types of conversations this is. The person who wanted to help but didn't know what to say or do sees and feels something they recognize, and suddenly feels more at ease, or at least human, and their expression softens. The gossip gets this flash of excitement in their face and eyes, like they will be getting 'the juicy' stuff next. And the guilty panic! It is so much fun to watch the guilty. Or maddening. But usually fun.
And the last, for me, very important thing that can happen, is that I have just had a connection with a real human being. I need that sometimes. Even if it ends there, it may have been shallow, but it was something.
For the gossip or the guilty, I just let them off the hook at this point. I give them something positive, hopefully about my wife ("you know, she has been working on crocheting this beautiful blue blanket, you ought to see it sometime"), or at least about me or my son("oh, I have been meaning to tell you, my son joined a baseball team, he is 4th string right field, and loves it"), change the topic to the weather or something banal, and find an excuse to move on. You may want meaningless conversation, it is nice sometimes, I just usually don't have time for it. But, whatever positive thing you told them, no matter who you are talking to, there is a real chance that it will get repeated. Maybe the same person will ask again, maybe someone they tell it to will approach you. And maybe next time they will be better equipped to actually care. Or not. But you probably didn't alienate anyone. And you would be surprised how easily [most] people drop their silly suggestions, once you have made it clear you heard them and thanked them.
For anyone who remains, give them a real conversation, which is what they really want (they don't actually care if your spouse follows their uncle's exercise program, they just didn't know what else to say). Me, I usually just launch into an explanation of the feeling I just expressed in my tone and body language, as best I can (which is usually pretty poorly). Sometimes, I skip that part, and go right into something along the lines of "you know, actually, there is something I have needed to ask..." But no matter what I say next, the conversation and its tone has changed. That is why it was important to have a list of things you need, so you can keep the conversation here, instead of slowly drifting back to awkward, where they fill the silence with nonsense about how they heard the sickness was caused by plastic in water bottles.
I have found that, once I have had a few real conversations with someone, I can much more easily gauge what they mean, and they are far less likely to bring up nonsense. As a matter of fact, I will sometimes even enlist their help, after a few conversations, in trying to stop the horrible suggestions. When I have 30 minutes with someone who has actually helped, whom I trust, I will sometimes try to explain to them exactly how much it hurts to have people try to armchair quarterback the one thing I have been pouring all of my time and effort into for years, the one thing that matters most, the one thing that I can truly say I am the world expert on. And you will be the world expert on caring for your husband. They never get it, not really, but usually they kind of understand. They already know you need and want help, and will probably feel the pain you are expressing. At least, they probably won't bring up stupid ideas again themselves, or act like they know all the answers.
Now, if someone is already in a deep or emotional conversation with me, before they bring up advice, "Thank you" isn't as effective. Not to say it can't be used in the middle of a conversation, just that it isn't useful unless you can actually elevate the perceived meaningfulness of the conversation. If it has already been deep, like when they were consoling me about a death in my family, and then out of the blue tell me I should put my wife in a nursing home and get on with my life, "Thank you" doesn't really cut it. It hurts more in those cases, a lot more, and elevating the emotion through physical contact (unless that emotion is anger, which I tried to avoid displaying, but would probably cheer you offering them a right hook to the jaw) doesn't cut off the discussion, which is your stated goal here. What I have found most effective might be seen as manipulative by some, and I am not sure how I feel about it. But it works.
Instead of reaching out, close yourself up emotionally. Get tears in your eyes (easy for me, whether the suggestion is just a dumb one or an offensive one, I was just insulted, for the hundredth time, this time by someone that I was opening up to and trusting, and you have a deep well of emotion you can't let out in front of your spouse anyway), say something along the lines of "I just can't..." (letting them finish the sentence in their imagination), and if they don't back down quickly you get out of the situation (go to the bathroom, to your car, walk outside). It is kind of the nuclear option, but it won't offend them. Probably. Some people, well... you don't need to be friends with everyone. So, yes, it isn't offensive. If overused, it is crazy, and will rightly label you as such. But if this is happening often, sincere conversations becoming people telling giving you unbidden trite or horrible advice on how to take care of your family, you probably need to rethink your choice in friends.
So, that is my suggestion. I know it is a little more than you asked, but I have found it works in the short term to cut off undesirable conversations, but more importantly for me also works in the long run to avoid isolating myself.