First, I wiped all the gunk out of the drain and drain stopper. After that, I cleaned and foamed the drain with a solution that removes the bad bacteria from the drain. The bad bacteria is the breeding ground for the drain flies. By removing the bad bacteria we're eliminating the breeding habitat for the drain flies. Targeting the drain flies alone won't solve the problem. To rid the owner of drain flies, I must also eliminate the eggs and the larvae that are hiding in the gunk.