Fungus Gnat larva grow in soils that are quite moist, usually too moist for the plants that are growing there so the first thing is to check your soil and only water when that soil is dry down to 1-1/2 inch to 2 inches. The easiest way to check that is to insert your index finger to the second knuckle (from the finger tip). You can use yellow sticky traps (these can be made from yellow poster board smeared with some sticky substance) placed around where the adult Fungus Gnats fly and they are attracted to those traps and held there so they cannot mate and produce more eggs that would hatch and become more larva.
Those Mosquito Dunks (BTI) can also work in severe infestations, but allowing the soil to dry is the simplest, easiest means of control, as well as the least expensive. Neem oil products do seem to work, but like many other products of this nature and given the rapid life cycle of these Fungus Gnats they may well develop immunities, as they have to other pesticides, to Neem products if they are used too much.

Thanks for responding. I have let me seedlings dry out already. There aren't many- but I don't want them to get out of control. I'm just wondering if neem is safe to use on seedlings, in case I do need to spray the soil, etc.

Thanks again.