they are great, and not just for fungus gnats.
it's good to kill them at different stages of life cycle at the same time -- look into any product with Bacillus thuringiensis Israelensis -- Mosquito bits and dunks are one brand.. searching for the bacteria may bring up a cheaper generic... a rose is a rose, in this case.
Anyway, this stuff has been used on our food crops for decades. You mix it in with your irrigation or top dress with bits and water in. It will kill the larva. your predator bugs will kill the adults... this bti stuff is good for killing most larva of any kind we'll see in the soil.
if the ladybugs run off, some yellow sticky trps will help kill the adults too, but obviously not a good idea with ladbugs around. Ladybugs will move on if not enough food.