We use a flame thrower (weed wand) for several days in a row if we have an infestation, then every week, or every time they are cleaned out, after that. The flame will get into every nook and cranny, but don't point it in one place for too long or you will burn the whole place down. Then we spread diatomaceous earth throughout the house and on the hens, working it well in under their wings.
Red mite always come back, even if you do burn the old house and buy a new one - we've done that once before too. 'The word' is that they stay in the old house when you move the birds, but in fact enough seem to hitch a ride on the hens to re-infest the new house.
We make sure the hens have a dry wood ash dust bath, or access to the bonfire site once it's burnt down, as the ashes seem to help keep down parasites, and of course they love a good dust bath.