The yellow sticky traps are great for keeping them under control. The actual gnats are not the ones doing damage. It's the larvae that eat at the roots. I had them bad in house plants and one method of wiping them out is to let the soil dry a little. Then water with a 4 to 1 water to hydrogen peroxide mixture. It will kill the gnats having a field day on the top layer of soil and the larvae eating your roots.
From the interwebs
Mix one part 3% hydrogen peroxide with four parts water. Allow the top layer of your soil to dry, and then water your plants with this solution as you normally would. The soil will fizz for a few minutes after application; this is normal. The fungus gnat larvae will die on contact with the hydrogen peroxide.
