it tends to happen when you grow stuff like mint indoors. The flies hatch when the plant is overwatered. Easiest thing to do is to pick some shoots off the mint, wash them well to get the sticky off and root in a glass of water then replant in fresh earth and a fresh pot. Discard the original.
I have also had great success with this stuff and pests on my orchids, it is safe for edible produce but you can't get it on its own, only with the feed.
http://www.qvcuk.com/...74/walk.yah.UKGD~U084