Fruit Flies

This worked for me, can't say which, or a combination of the methods worked:
 
Pour a little ammonia down the drains every 2-3 days.
 
Make traps - fill custard cups with about 1/2 cup white vinegar, secure the top and sides tightly with aluminum foil, then punch holes in the top with an ice pick.  I left a couple on the kitchen counter, and one in the bathroom.  You'll be tempted to peek but will get discouraged.  Wait 4-5 days.  You'll be amazed at how many you trap.  You could probably go on forever but the thought of all those little fly bodies floating on top was kind of gross, so I threw it out, and started a second application.  By the second round, most were gone, and I didn't catch many after that.
 
I keep a spray bottle of glass cleaner, in the kitchen when they land, give them a spritz, and wipe down with a paper towel.  You can catch a few that way, and your walls will be sparkling.
 
  
 

Wow, I wish I had noticed this thread last October when we also had an infestation. My daughters blamed it on their brother's room! I couldn't find fly paper but found a yellow jacket trap and used it. It worked really well. Next time I will use some of these suggestions.

"Time flies like crazy, but fruit flies like bananas"   (the late Townes Van Zandt, during a live KFAT-Gilroy radio show during the time San Jose (CA) was being sprayed at night by air and ground for a very serious fruit fly infestation a few decades ago)

Check and see if there is a Natural Insect Control business close to you. There will be a "something" that fruit flies like better than you and your store.