Getting Rid of Fungus Gnats?

If you do have fungus gnats in your home, the management method you used, spraying in the air, is not effective. The main problem is that fungus gnats spend most of their lives as larvae buried in indoor plant soil and the ones you see are a few very short lived adults. The immature gnats live in plant soil for months so you must improve the soil to kill the immature gnats before they fly.

Please see the suggestions below to try to control your problem.

Management
Fungus gnats are typically harmless to healthy plants - and humans - but can inflict extensive damage to seedlings; their presence can be indicative of more serious problems. In houseplants, the presence of fungus gnats may indicate overwatering; they may be feeding on roots that have sat in drain water too long and are thus rotting, or the gnats may be attracted to fungus growing in saturated topsoil.

Consequently, allowing the soil to dry may reduce their numbers. They are sometimes also managed by placing a layer of sterile sand or indoor mulch on top of the soil around plants; by introducing Hypoaspis miles mites or applying the biological larvicide Bacillus thuringiensis (subspecies israelensis) to kill gnat larve; by drenching the soil annually in an insecticidal soap; or by applying detergents and nicotine from tobacco brewed into a toxic tea.
Insecticides are considered an important control option in some commercial plant production but generally aren't recommended for fungus gnat management in and around the home.
University of California Davis
www.ipm.ucdavis.edu/.../pn7448.html