I would remove the aspen (you can put it in a bag and keep it for later use). Spray some paper towels with PAM and use those as your substrate until you know your snake is mite free (after they dry of course). Use white paper towels without print. The reason for this is so that you can see the mites easier. Your enclosure may still have mite eggs that will hatch and try to re infest your snake. The PAM will kill them once they hatch but not before. When you no longer see any mites on the paper towels, you can switch back to aspen.