If you want to kill the fruit flies quickly try to put out a bowl of vinegar, some sugar and a little bit of dish soap. I found this to be super effective.
The sugar and vinegar kinda mimics the scent of rotten fruit which they are drawn too and the dish soap will destroy the surface tension, so that they just drown.
I've tried that and found that they will drown even without the dish soap, but the problem for me is getting them to go for the liquid instead of just hanging out all around it.