If you only have a few plants you can take and rinse the roots off remove all the soil and repot. I probaly wouldn't do it if they are in flower but veg will be fine for sure when you repot them put a layer of sand on top 2" thick to prevent them from getting back in. also gnatrol works good with 3 applications 5 days apart that will kill the larve the stick traps mention before will help with the adults but Pyrethrin foggers work best. My 1st grow I had them bad so bad they were stuck to the nugs and died it was nasty. I took a noodle strainer and a trash bag put the weed in the strainer the strainer in the bag then a bungee cord around the top of the strainer to keep the nugs from getting out into the bag than shake it the gnats fall off the buds through the strainer and stick to the trash bag do to static electricity. Cleaned up you couldn't even tell I still never smoked any just the thought of it. I moved it to a friend at a discounted price.