They aren't fruit flies, they're fungus gnats. If you look at them more closely, you'll see they look more like tiny mosquitoes than they do like fat fruit flies. You'll need to treat the bark to kill the larvae. There are many fungus gnat remedies posted on these boards, but I have found the most success with a product called Mosquito Bits. You can buy it on Amazon or in garden centers in granular form. The treatment takes a few weeks to kill all cycles of the larvae. In the meantime, allow the media to get as dry as possible between watering a without damaging the plant.