2 Answers

Get a bowl. Place a piece of banana inside.  Cover the bowl with plastic wrap.  Poke holes in the plastic wrap with a tooth pick (about two dozen)  Leave it on the counter.  

You can take the bowl way outside and let them go each morning if you like or place the bowl in the sun ... they die after a very short time (sad though... poor bugs).

You may have to do this a few times ... cleaning each day as you go. It works. 

3 parts vinegar to 1 part Dawn dish detergent. Other brands don't work as well. Mix it together and

set it on your counter. You will be amazed .  I put in a shot glass by the next day the bottom is covered in dead fruit flies. Then I mix up a fresh batch until I have no more