the way to prevent fungus gnats is to keep them from a damp medium. One thing you can do is cover the soil with paper plates.

Hydroponic growing isnt immune either! They will find the water source and go right into the clay pebbles to mate. as long as they can find a moist environment!  (freshly watered soil/ coco / a wet block of rockwool / a cup of water. They are all a perfect environment for fungus gnats) I have even seen them make their way INTO my buckets!

sticky traps help. Ladybugs can help. but you still need to isolate them from that moist environment