Drain flies are particularly difficult to get rid of, as they breed quickly and hide in hard-to-reach places. To eliminate them, you need to use a combination of methods that target both the adult flies and their larvae.
A multi-tactical approach is often needed to decimate the population until you wipe them out entirely.
Clean your drain and pipes thoroughly, as this is where the flies lay their eggs and feed on organic matter. You can use a pipe brush or snake to remove any sludge and debris, and then pour boiling water, or a commercial drain cleaner to sanitize the pipes. You need to remove and scrub any surfaces that may harbor drain fly eggs, such as the underside of the rubber drain shield in your kitchen sink. Make sure to scrub them off first because even undiluted bleach alone won’t kill them.
These flytraps are cheap and work very well. I hang them in multiple strategic locations and they get covered quickly with flies.
Pour vinegar and a drop of dish soap in a glass covered with plastic wrap punctured with holes is a great method to attract, trap and drown large numbers of drain flies. The flies will be drawn to the liquid and drown in it. My persnickety flies prefer Merlot over vinegar, but thankfully they don’t insist on an expensive vintage.