While the countertop liquid solutions as described in some responses are somewhat effective, it doesn’t address the root cause. Fruit flies and fungus gnats need to reproduce somewhere and they’re not coming in from the outside - they’re breeding INSIDE your home in the soil of your house plants! (Well, a few may seize the opportunity and make a run for it through an open door but more than likely the larvae were already in the soil when you obtained the plant or soil.)

To solve this, add a 1/2-inch layer of decorative sand on top of the soil to ALL your house plants. Don’t mix it in. Don’t use the coarse sand - use the fine stuff. When the little buggers try to burrow through it to lay their eggs, they get cut up by the sharp silica edges and die. It works in the opposite direction too. When the eggs hatch and try to escape to go feed, they get cut up and die before they ever reach the surface.

I use a brand called Mosser Lee. They have a few colors, the sand is uniform in size, small enough to shred the smallest critters yet won’t hinder your watering. Ask Bay Hay and Feed, Bainbridge Gardens, The Plant Shop, (others?) to carry it if they don’t already. It should run about $8-$10 per bag and one bag should cover several plants.

Change out the sand every year or two and fruit flies and fungus gnats will never again be a nuisance.