for a boa, pick up some hartz flea and tick powder for cats. clean out the tank,trash all the branches (trust me here). sprinkle the powder on the bottom of the tank, then layer paper towels over it. Do not give a water bowl for the first 24 hours. then clean the tank again and give water for a few hours. The remove and continue this cycle for two weeks. it will take care of the issue. Its time consuming but its one of the more affective methods for removing them. You do not want your snake to ingest the powder so be careful with it. Also, if you have geckos, get them to the other side of your house because mites are very hard to remove once you have them on geckos.
vegetable oil is great for geckos because you can easily spot the mites on your animal but for snakes the mites can hide extremely well between scales (mite eggs included, and particularly around the eye scales and heat pits).
pm me if you have any questions.