Multi pronged attack is right...... three to four Full sterilizations, implementing precautionary measures...... even if you think they are gone...... do it again.... Larvae is also tracked in via fruits, veggies, especially potatoes and bannanas....... Fungus gnats Hate Citrus
If your water is basic, using lemon juice to ph the water also helps deter the buggers from ever coming around (though cinamon does a good job too, it will eventually break down and filter out through your media)...... The best Preventative measure for fungus gnats and the many other harmful bacteria, fungai, and other pests, is Not Overwatering! Not letting the media get too cold, too hot, or suffocate...... and not exposing the media to foreign materials.....
For example.... I got a pesky mouse that keeps delivering acorns into "special" pots in my flowering room! Just the Maui Skunk and the Twilight.... interesting huh? Well, the acorns coating (protective shell) releases bacteria, and breaks down into toxic sediment deposits in the soil, killing roots, and ruining the microbiotic environment!