There are certain people, especially family members you see from time to time or other close relatives, who simply don't know where to stop when offering you something. Usually it's about drinking alcohol, eating cake or other things I must do because everyone else does that too.
When they ask me whether I want it, I just say "no, thank you" and then it starts... it turns out it wasn't a polite question but actually an order so they start digging why I don't want to drink with them or why I don't eat this delicious cake?
It never stops until it's too much for me and I end the conversation with "It's enough now, I won't drink it!" or I simply throw in "Stop, it's not your business".
I know they are the ones being rude by insisting on me doing something I don't want to but still, I wouldn't like to end it this way and prefer them leaving me alone without starting an argument or insulting someone with a pretty impolite response... or maybe there is no other way?
I don't want to explain myself why I refuse to do something (like many answers suggest: one, two, three, four - please don't get me wrong, these are all good answers, however not for this scenario). It's not their business and I don't want to discuss this with them.
I really don't know what else to say but explaining my self like: I don't like it, I already ate, I'm tired, I'm allergic (not really, but sounds like a good excuse), or even I prefer not to drink from this cup because it looks dirty etc. etc. which usually doesn't work well because they very quickly will crush it and come up with an anti-reason.
In case there is no good way of handling this without excuses maybe there is some ultimate universal excuse that cannot be undermined easily and will stop these pointless conversations before they escalate?