To get rid of drain flies, remove the slimy film inside the drain where they breed, not just the flies you can see. Scrub the drain walls and the sink overflow with a stiff brush, follow with an enzyme drain cleaner, and repeat for several days until no new adults appear.
Drain flies, also called moth flies, are small, hairy insects about 1/8 inch long, according to Utah State University Extension. They lay eggs in biofilm, the bacterial slime that coats drain walls and the inside of the P-trap, and the larvae feed on it. To find the source, cover each sink, tub and shower drain overnight with clear tape, sticky side down, leaving a small gap for air. The drain whose tape catches flies is the one to clean first.
Bleach and fly spray kill the adults they touch but leave the film, so a new batch hatches and the flies return, one reason bleach is the wrong tool for a drain. Boiling water has the same problem and can soften some plastic traps. Rarely used drains, such as a guest shower, need the most attention, because the film thickens where water sits. Larvae in a toilet bowl are a separate case, covered in the small black worms in a toilet.
One honest limit: brushing only reaches the first few inches of a drain. If flies keep coming after every drain is scrubbed and treated, the breeding site is out of reach. It may be heavy buildup further down the line, a floor drain whose trap has dried out, or moisture around a broken pipe beneath the slab, which the same extension page lists as a breeding site. Deep buildup needs professional drain cleaning, and a broken pipe needs a camera to find it.