Mites are a pain in the butt, sorry to hear your boa has them.
Buy a can of Provent-A-Mite and follow the instructions on the label. I battled our mite infestation for six months with various products, to no avail. Then I bought a can of PAM and they were gone within the week. I swear, it saved my sanity. (Just remember that mite sprays are poison, and it is imperative that you follow to directions precisely to keep your snakes safe.)
Also, soaks help kill off mites short-term, and I think it soothes an infested snake to have at least a number of them drop off.