Pour boiling water down the drain daily for several days
Scrub the drain, stopper, and overflow opening with a stiff brush
Clean inside the drain pipe with a pipe brush
Remove and clean the P-trap if accessible
Use an enzyme-based drain cleaner overnight
Eliminate standing water and damp organic buildup around the sink
Fix leaks and dry wet areas under and around the sink
Clean garbage disposals thoroughly if present
Cover drains when not in use
Set sticky traps near the sink to catch adults
Repeat cleaning until flies stop appearing
Call a plumber if the problem persists or the drain is damaged