Try baking it. I had a springtail problem for a while, and it was one of the solutions that popped up in a search. It was posted across many pet turtle-related forums.

I didn't try it personally, because the problem solved when I solved the other problem my few infected pots had.

Here is a link that might be useful: http://www.healthrecipes.com/soil_plants.htm

Baking any soil, or microwaving, will cause a stench in your house that will be difficult to get rid of. Pouring boiling water over your mix would be better, if done outside.

And that stench means it was over-heated and has released materials that are potentially toxic to your plants.

I had a nearly full bag of seed starting mix left over from last year. It had been in my garage (half-open) ever since. I didn't want to waste it, so I put it in the freezer for a few days. I used it to start some seeds after Valentine's day and so far, not a single seedling has dampened-off. No fungus knats, either.

Tap water should be hot enough if you rinse it thoroughly after hot water has reached the faucet. I'd just leave the bag wide open in the sun for a few days until it is completely dry. Fungus gnat larvae can't survive in dry soil. You could also spread it out on cardboard to let it dry. Of course with weather like we're having that would be impossible here right now.