I had quite a heavy infestation about a month ago. I cleaned out the house (6x8 shed, so not small!) and sprayed with poultry shield twice- 4 hours apart. Make sure to get in all the crevices, end of perches, doors etc etc. Let it dry and then covered the whole house in diatom powder and minimal bedding. That got rid of a huge amount of them but it's still not totally sorted, the problem areas (door and frame, nest box lid) have been sprayed every week since and I've just about got on top of it now. I don't think it will ever be eradicated but I'd like to get it under control.
I also dusted the hens in diatom powder especially under their wings and put it in their dust baths.