What a nuisance! Fruit flies are the worst. One summer they were so persistent, I felt like I had some neurological disorder, swatting at the flies, shaking my head at flies so small no one else could see them.
Here's the Simple Solution: Take a small saucer, pour in apple cider vinegar. Cover with plastic wrap, stretched tightly. Poke holes, a kabob stick will work, or a fork.
The idea is that the flies are attracted to the smell, crawl in the hole and are trapped. The only note I have to add, is open and pour the vinegar at the same spot where you will place the plate. Otherwise, the smell distributes on the path you carry the plate, or open bottle. Then the flies are everywhere!
Here's the Second Simple Solution: Keep a bottle of Windex by your kitchen sink, or bathroom mirror. Have paper towels handy. Every time you see one land on the window, spray it. Wipe and dispose. You'll be amazed at how fast the problem is resolved. For some reason the Windex spray bottle works best, any blue window cleaner will do.
(I have mentioned that I spend a lot of time at the kitchen sink? But at least the fruit flies are gone.)