I know this is not very ecological or 'green', but it worked for me. If you have a garbage disposal, take a can of Raid or Black Flag and pray into the disposal, then cover the drains with a plate or tight lid. I left it overnight and in the morning, ran really hot water in it and ran the disposal. It worked! If anyone has a natural remedy for the Raid, I would love to hear it. I don't care for the stuff!
They are probably fruit flies.Helps to make sure that overly ripe fruit isn't sitting out. Pour bleach into your sink drains. I don't know why it works but it does!
Determine first if they're fruit flies or gnats. I found a very effective fruit fly trap at Wal Mart for about 5.00. Got rid of them in about 3weeks instead of the 3month battle I'd been waging! And no viewing the dead ones! That was gross! It's brightly colored and in the shape of an apple !