Fungus Gnats in Closed Terrariums?

Fungus gnats are in my closed top terrariums. I have tried every method that I have found on line (yellow paper with fly strips, and I have killed a lot, but not all). I found out that plants brought inside can have them so I took them all out on the front porch and put sand in the top. I discontinued watering on top and put a tray under them to water. (Someone suggested vinegar and that works.) I also put my potting soil in the freezer. I haven't put it in bake yet but will try.

I have reworked four, washing the soil completely off roots, and cleansing the jars and rocks. After reworking two recently, one of them I discovered has a few gnats. I haven't tried insecticides because I had planned to sell them, but until I solve the problem completely, that's out. Help! I am working hard and everything works but not completely. If insecticides are used, do I spray the soil or all the plants?

By Barbara