@Star Dog
I've been using a combination of mosquito dunks and spinosad to kill the larvae.
And strong airflow at pot level and sticky traps for the adults.
I had a horrific infestation a while back Because my fan died in veg area, I don't know when but I didn't replace it straight away.
Apparently that's all it takes to make life easy for them and take over.
Some of my sticky traps you couldnt even see the yellow, they were covered.
I have a lot of spinosad on hand so googled it and it is affective as a soil/coco drench against fungus gnat larvae.
Something else I've not tried is microwaving the coco in small batches to kill any eggs or larvae before using.