Put an inch of clean coarse sand on your soil. Fungus gnats can't reach the soil, so any eggs that do get laid, hatch only to starve on the sand! (If you use beach sand, wash it well first! Cannabis is not fond of salt.) The easiest way to wash sand, is to go out on the lawn, half fill a 5 gallon bucket with sand, and stick a running hose in it. The dirt will be washed away in the over flow. Just push the hose up and down through the sand. It can be kinda fun to do if you're stoned!

A nice little plus, if you use white sand, light will be reflected back up to the plant! A couple of sticky traps will get the adults.

And I don't know about your brand of neem, but mine STUNK! Clean sand doesn't smell bad!- Granny:hippy: