Pour boiling water down the drain several times
Scrub the drain opening and inside the drain with a stiff brush
Clean the drain stopper, overflow holes, and surrounding area
Flush the drain with a mixture of baking soda and vinegar
Use an enzyme drain cleaner overnight
Remove and clean the P-trap if accessible
Eliminate standing water and leaks near the sink
Empty and clean trash cans and recycling bins
Keep food scraps and dirty dishes out of the sink
Cover the drain when not in use
Set apple cider vinegar traps near the drain
Repeat cleaning daily until flies are gone