The easiest way to get rid of fruit flies is to get rid of their food source, but if you have no source then you have to resort to catching the ones that are already out there. They're very small and annoying, though, so we had to set a trap.
I took a bowl of cider vinegar with a piece of mashed-up banana in it, covered it with plastic wrap and poked some small holes in the plastic. Don't make them too big, though. I poked a lot of small ones rather than a few large ones.
Put the bowl in the general area of where most of the flies seem to be lurking, and leave it. They'll go through the holes to get to the fruit and the vinegar, but they won't be able to get out. This will trap most of them, and in the meantime you can give your kitchen a good cleaning to make sure that there are no sources of food that they're finding.
I put a bowl out and in about an hour there were a bunch of them inside, so it really does work. It's a lot easier than trying to catch them individually, too!