The easiest control for fungus gnats is to allow the soil to dry out completely between the times you are watering them, which may actually be the best thing for the plants anyhow. The second option would be to cover the soil in these pots with some material that prevents the flies from getting to or from the soil, such as a thick layer of spagnum moss or some screening. The third option would be to use an insecticide such Talstar P. Talstar P is a concentrated liquid insecticide that is diluted with water and then poured onto the soil and sprayed onto the plant itself. Talstar is odorless, very effective, and is labeled for over 100 different insects. It is also what most professional companies use to spray outside and inside homes to kill insects. It is safe for use on plants and grass…..however it should not be applied to anything edible. We recommend starting with letting the soil dry out between watering as this offers the fastest control in most cases.
Here is a link to the Talstar for you to read further: http://www.domyownpestcontrol.com/talstar-talstar-one-p-97.html