What you need to do is go to a farm/feed/garden store and pick up some Sevin 5% Dust. It's in a red container. Dust the birds with it, either by putting some in old nylons and using the nylons as a powder puff all over their body, or by filling a small bag with Sevin, putting the bird in leaving just its head out, and gently shaking the bag so the dust goes all around the bird. I like the nylon method, lol.
Then, clean out the coop. Clean it well. Use a gentle soap and water. Dust everything with the Sevin (the walls, floor, roosts, literally everything), put bedding in and mix the bedding with Sevin. Wait a week, and do it all over to kill any that might have hatched in the meantime, since Sevin does not kill eggs, just adults. And make sure to do all your birds - if one has it, they all do.