I'd use a few things all at once. I'd group water in BT from watering in mosquito dunks, and then let them all dry out. When dry, I would cover the top of the soil with diatomaceous earth, and would bottom feed pots with SNS 203, and also foliar with SNS 203. I'd spray goGnats (cedar oil) around the place, they hate it. I'd also water in a way as to break their cycle. A quick search, will get you there, I don't recall the specific lifecycle intervals. Good luck in your fight. I'm not in a damp basement, so it's not been that hard to knock them out.