This is a common complaint among crabbers and a very frustrating situation. Some mites can be very harmful to your crabs while others are actually beneficial in keeping the tank clean.
There really is only one way to successfully rid your crabitat of mites. That is by introducing predatory mites to the crabitat. Repeated bathing, boiling and sterilizing is not proven to work and all that bathing will stress your crabs.
For more information on mites and dealing with them please read the following article: