Sorry for your bad luck! You should be keeping the snakes on white paper towels for some time to come, as mites tend to come back in generational waves. You need
to keep checking & re-checking.
If your Repti Bark was near any snakes that now have mites, you might want to bake it or toss it, out of an abundance of caution (in case some mites drifted into it).
When you soak your snakes in mild soapy water (like Ivory), use warmish water & stay with them for 20-30 minutes to make sure the mites drown. But, they can still
survive on the snake's face where you cannot soak, so keep in mind that while this helps, it's just part of the solution. It helps get rid of many mites quickly, but not all.
Make sure that NO bleach smell remains on any hides...& never use bleach on anything porous (like wood/branches) because the fumes are toxic & will not
come out. (actually, I think most people throw out any cage decor that's porous...it's asking for trouble, mites hiding in crevices?)
Happy to say I haven't ever had to use Provent A Mite so I'll let those members who have give you pointers on that, except to say that it's important to follow
the directions carefully, otherwise it can harm your snake.