Get some sticky traps. Fungus gnats like the yellow ones. This will not get them all but will greatly reduce the numbers. A pyrethrin based spray is also effective on the flying adults. Some products containing Bt (bacillus thuringiensis) can be used to control the larval forms in the soil.
Prevention is key. Buy good quality potting soil or make your own with some compost, perlite and peat or coco.
try spraying you soil with a mixture of some water with some dish soap thrown in. I have used that with success before. Someone else I know uses DE dusted on the surface of the soil.
Perfect timing... One of my trusty houseplants (and by trusty I mean permentantly affected to some degree with scales) just yesterday errupted into a fungus gnat pit. I have NO idea where they sprung from. I've had this plant for a few years now with no environment changes.
In yesterday's search I found various suggestions.. the two that stuck were Provado spray (not sure what's in it, but it might be a little strong for seedlings) and peppermint tea. It sounds like the tea doesn't necessarily kill the flies, but it at least slows the fly output and eventually stops the issue.
Good luck! I'm in that boat with you..