To get rid of flies in the bathroom and kitchen sinks, inspect the area for any breeding sites, especially in the drains, and thoroughly clean the area of any organic debris. Once all breeding sources are gone, use an insecticide containing pyrethrin to kill any adult flies.
It is crucial to remove any rotting organic material from the area to stop breeding. Drain flies live off the debris that builds up along the inside of drains, and to remove these breeding sites, you must use a drain cleaner that destroys organic debris. Check nearby areas for leaking water, which can provide another source of breeding material beneath floors.