(C/P-ing from a past post about fruit flies.)
OK, so every damn thing you find on the Internet will have you building some kind of contraption involving several of the following: wine, dish detergent, vinegar, plastic wrap, sugar, water, and a bowl or glass of some kind. None of these have ever worked for me. They may trap a few flies, but if there’s one thing that fruit flies are, it’s prolific. There are more of those fuckers than a homemade fly trap can catch.
First things first: you need to find where they’re living and eating (and breeding). This is almost always going to be some kind of food source. If they seem to be coming from the drains, they’re probably living off of the organic crud in your pipes. Drain volcano and/or commercial drain cleaner is a good place to start. Duct taping the sink drains so they’re air-tight might work too, as long as you don’t need to use the sink. Keep in mind that they can live off of food crud that lives on your dirty dishes. I’ve found colonies of them living off of a single piece of dog kibble that rolled under the stove.
Once you’ve found their home and disposed of it outside of your house, you need to clean some shit. Using a bleach solution, wipe down every possible surface to make sure there’s nothing for them to live or feed on.
Some people swear by freezing them out, so if it’s cold where you are and you can stand it, you might try that. The most important thing, though, is to find where they’re hanging out and to make sure there’s nowhere for them to make a new home base. It may take a day or two for their numbers to noticeably dwindle.
Also flypaper. It’s available at most hardware stores, and it’s really satisfying to watch those fuckers fly onto it and get trapped.