Provent-a-Mite is the stuff. Follow the directions on the can. Do not spray this stuff on the snakes. You can spray the cage and substrate. Once dry you can return the snake. It does not take long to dry. Once dry it will kill any mite that comes in contact with it for at least 30 days. Spray the substrate whenever you change the substrate.
To remove the mites from the snake is different. You can wait because they have to come off and then they will come in contact with the substrate and die. Or you can use Reptile Relief which is safe and effective. Another thing you can do is soak the snake in water with a few drops of dish soap. I like to soak for about 6 hours.
Ultimately Provent a Mite is what will kill the mites. You can also spray it on the rack, stand, floor or anything else. You can spray it monthly as a preventative. Get it from Pro Products. Call them and he will tell you anything else you need to know.
Dan