This may sound weird but I have had success in treating for mites using a combination of treatments. I spray my cages with prevent-a-mite but I also treat my snake to kill the ones on them. To treat the snakes I used olive oil (I assume regular vegetable oil would work too) to coat the snakes. I let it sit for about 15-20 minutes and then wash them off with a little bit of dish soap and water. You need to make sure you get all the oil off because it's bad for the snakes to leave it on. Doing both of these things I've had success on wiping out a mite problem fast. I've had to treat for mites twice now and in both cases I only needed to treat the snakes once and then did a followup cage spray 3-4 weeks later to make sure I got them all.
Edit:
Once they are gone they usually stay gone unless you bring in another snake that has them.