so instead of spraying the substrate just use cypress. (although using PAM is still a good idea) If i am not mistake it has a naturally occurring insecticide that is safe for reptiles. But spraying the enclosure is still a Must.
As far as what to do when you have an existing mite problem... there are many things and i declare full out war.
soak them in a tub of water with a few drops of soap as well everyday.
Newspaper and paper towels ftw as bedding can also be used, and clean the cage every 3days with windex. after that spray it down with the anti mite spray(PAM or other). then sprinkle tiny amounts of anti mite powder(again if you have it or feel it is needed).
here is what i used when i got the burm i had and had to get rid of them because i couldnt get PAM at the time, and even if i had it i would still have used these items as well because i hate mites...
http://www.reptiledirect.com/bensonandsonssnakeoil.aspx (not really all that needed but it helps)
http://www.amazon.com/Nature-Zone-Mi.../dp/B0002ARSKS
http://www.amazon.com/DeFlea-Reptile.../dp/B0002QIII8
you must be super diligent in this and even if you think it is safe to stop treatment, keep doing it for another 3-4weeks at a minimum
-todd
quarintine the animal to a completely different room as the mites can climb out of most tanks!