Once you get fruit flies they are hard to get rid of. I have never totally eradicated them. Lots of great advice on the internet I'd google it....I find the old fly tapes catch piles..

Good luck

SCG

They are probably fungus gnats not fruit flies.No matter what they are you can use mosquito dunk bits (they sell these) to get rid of them. Exactly how to use them though is unclear. many methods are suggested,
The Mosquito Dunks product contains Bacillus thuringiensis israelensis (BTI for short).

You have fungus gnats not fruit flies. Fungus gnats are easy to get rid of. The gnats only appear to mate ,lay eggs and die. They spend most of their lives under the soil. First as eggs and then as larvae eating debris in the soil and plant roots. Get rid of the eggs and larvae and the gnats soon disappear.

To get rid of the eggs and larvae mix a 1/2 teaspoon of insecticidal soap or dish soap in a quart of warm water and water the plant. do the same to any other plants you have in the house. Fungus gnats aren't particular where they lay their eggs and other plants will be infected too.The soap won't hurt the plant but it will kill fungus gnat eggs and larvae