I worked in pest control for about 5 years, and I figured someone here might need some advice on how to deal with bugs and mice. So if you've ever wanted to know, now's your chance.
My biggest recommendation, right from the start: Never pay for pest control. You can do a better, more ethical job than any business ever will.
I want to edit to say, my inbox is always open for questions too.
So I had a problem in a house I used to rent where we would get flies from our drains. Little black fuzzy fruit fly looking things. Being in an apartment also presents its own issues because your neighbor could have a banana farm going in their kitchen.
I would first try to identify that they are fruit flies, since it sounds like you've taken care of their food source. Once you're sure where there coming from, you can spray the areas with a residual pesticide .
Does boiling water down the drain work as well?
IDK about just water, but nothing will survive a nice round of drain cleaner. Of course don't just go dumping random chems in the sink, but drain cleaners are usually some kind of horrible acid or intensely strong base. So that'll kill anything down there.