As the man said above, let your soil dry out inbettween feeds, not compleatly but enough that the top inch or two is nice and dry. Once this is done you can also chuck a couple of thick slices of potato on the top of the soil with moist side down, any living lavae will move to the potato after 12 hrs or so chuck the potato in the bin, if you can when doing this move them out of the grow area till you throw out the potato and clean the grow area throughly.
Ignored fungus gnats are not only bloody annoying but the lavae can damage roots badly.