If it were me, I would strip out the entire cage and use newspaper or paper towels as a substrate. I'd be willing to bet that mites can lay a crap ton of eggs in a loose substrate like aspen, making them more difficult to get rid of.
Here's exactly what to do with the Provent-a-mite:
1) Remove the animal and the waterbowl from the enclosure. I'd keep the animal and water in another room. Remove other animals from the room and cover any fish tanks, if you have any.
2) Spray down the enclosure- hides, walls, substrate, everything - don't saturate it, but you at least want it to be slightly damp.
3) Wait until the enclosure is COMPLETELY dry (its only dangerous to reptiles when wet) and all vapors have dissipated. Now would be a good time to open a window or something, just to be safe.
4) Once the enclosure is dry, you can put the snake and the waterbowl back in. Any remaining mites, eggs, larvae, etc, are doomed. Try not to remove the substrate for a few days at least to. Better to let a urate sit there for a couple days than to waste a treatment. Treat again in another few weeks just to be safe.
Hope that helps!