The most effective way to rid indoor plants of these gnats (called "fungus gnats") is to regulate the watering schedule. Fungus gnats are generally a problem in soils that are over-watered or that have too much organic matter decomposing in the soil.

Allow the soil to dry between watering or use a moisture meter to help you decide when the plants are in need of moisture. Remove any debris that may be decomposing on the soil surface such as old leaves and stems and add them to the compost pile. The gnats should stop being a problem once you have adjusted the watering conditions.

— contributed by Garden.com