These are sort of a "left field" kind of thoughts that might work for the right person or situation, or they may fail miserably. This is a bit of a frame challenge, since it's suggesting you to look at this from completely different directions.
They've offered to watch the store for you. In that same conversation they said they don't have anything else to do all day. You also mentioned that you are short staffed and need to hire people. That comment about not having anywhere else to be might be a clumsy way of asking you to hire them.
This all depends on their demeanor, sometimes age, and whether they will actually close their mouths and work. When I say "age", I mean are they retired and don't want to work anymore, are they a teen and don't have any summer job? Summer should be over by now, so teens are likely in school and early 20-somethings might be in college. And just because they talk a lot, it doesn't mean they can't work while they talk or won't be a good waitstaff, since they'll be good communicators. If they just blabber on all the time about anything and never get anything accomplished, then yeah, don't bother.
This at least temporarily solves the issue of them taking up your "piddling number of seats". Maybe they become a great asset, or maybe you fire them 2 weeks later.
Who better to become an employee than someone who actually likes the product and the business? What better way to spend the day than to be where you want to be and get paid for it? You can teach someone how to run a cash register, clean tables, or dip ice cream/gelato, but you can't teach loyalty. Companies are being shown that employees who are loyal to the company actually make better customer service, so why not start out with people who are already loyal?
This is entirely a judgement call on your part, mostly due to whether you can stand being around them for a full shift or if they are reliable and trustworthy.
This takes some time and some effort, but if you can get to know them and maybe trust them, you can leave the shop to them for a few minutes while you run a few errands. Also, this way they become people you want to see, instead of adversaries that need to leave when you're done using them for revenue. That's putting it a little harshly, but that's how many businesses are run even when not a restaurant. Building a real relationship with your customers makes a lot of things easier, including asking them to vacate when you are busy. They may even take more notice of your other customers and voluntarily leave if they think they are in your way, because they have become invested in your success, not just involved.
I actually liked parts of Spagirl's Answer, except for where the point was to become annoying so they'd want to leave and possibly never come back. Pulling up a chair and joining their conversation isn't a bad idea, if your intention is to actually get to know them. You may find out something that changes your mind about them being in your shop. Maybe you realize they are avoiding something at home, maybe they think you're cute, maybe they love the smell of only your shop, maybe you aren't an ass and forcing them out like other shops, maybe your shop reminds them of their childhood, or maybe they just didn't realize they were in the way. There are 1000's of other things that could explain them staying, including them being SOBs that just don't care or are waiting around for a good time to rob you. You won't know until you get to know them.
Let them be
As I mentioned in a comment on another Answer, people are more comfortable with entering a shop that has people in it than an empty one. An empty one suggests unpopular, not good, or simply the staff will converge on the new person. Having butts in seats actually makes the place more approachable. Also, being empty can give the vibe that you aren't open, even if you have an "Open" sign.
This link is all about getting people into your store, because people being in your store actually brings even more people to your business. People being in your store acts as immediate advertising, because people will want to know "why are there so many people in your store". If your store draws a crowd, it must be good, right?
As a manager of a small computer repair store, who was also the only computer tech, I had people say they had come by the shop but didn't see anyone inside, so they kept walking. They didn't come in until they saw me at the front desk talking to another customer. My store was set up so that the repair area was in back and I was the only employee, so the shop looked empty most of the time. It's true that many people came in anyway, and we had a door chime to let me know about it, but I'm sure there were people that never came in due to the emptiness. I had one customer specifically ask if I was open, since there weren't any other customers in the shop.
Again, all this depends on you and them. Are you willing and able to spend the time and effort to do any of this? Are they? That's something only you can know. No judgement here. Running a business takes a lot of time, effort, and concentration. My suggestions take all three of those, so it's totally understandable if you decide not to try my suggestions.