I got ya man..
Head on down and buy yourself a box of panty hose. Make sure while your in the womens isle that they see you trying to look through all the leggings and give you dirty looks.
Now, go back home with them. Hopefully you got the extra xxx size ones. If so, you might be able to stretch it all the way around the pot, and zip tie around the stalk.
If not, you'll have to get crafty, and cut up a liner, and tape it around the top and the drain holes of the pot.
Now the lil bastards cant fly in or out of the soil, and in a few days theyll be dead.
Plus, you can still water as usual, right through the fabric.
Good luck.
P.s.: if you wanna see something really cool, poke a straw down into the dry pot of soil thats infested with gnats. Then, hook up an e-cig device with ultra potent peppermint flavored nicotine e-juice, and pump the vapor right into the pot of soil.
Watch all the gnats and larvas squirm to the too, fly away, and die!
Then put panty hose over the pots, lol.