There are several ways to get rid of fruit flies in your drain, according to the search results:

1. Boiling water: Pour boiling water down the drain to eliminate drain flies. Boil a medium-size pot of water once or twice per week, and pour down and around the drain.

2. Baking soda, salt, and vinegar: Combine 1/2 cup salt with 1/2 cup baking soda and 1 cup of vinegar, and pour down the drain. Leave overnight, and finish by pouring boiling water down the drain in the morning.

3. Drain cleaner: Use a store-bought drain cleaner like Drano or Bio-Clean. These chemical unclogging solutions will clear out the drain and pipes and help eliminate any materials or debris that may be creating a breeding ground for drain flies, thus preventing new eggs from hatching.

4. Fruit fly trap: Create an apple cider vinegar trap using a small dish covered in plastic wrap. Poking holes in the plastic wrap will lure flies in but prevents their escape.

5. Clean the drain: Use a pipe brush to scrub around and inside the drain. Pour boiling water down the drain to loosen rotting matter. Use a metal pipe brush to clean the pipe lining. Follow with more boiling water.

Its important to keep your sink and drain clean and dry to prevent fruit flies from breeding. Wipe your sink dry whenever you arent using it because fruit flies need only a little bit of moisture to breed. If these methods do not work, you may want to consider scheduling a professional drain cleaning.