You need to get your buns to the vet for some ivermectin. You can get it from pet shops but it's not very strong so you end up using loads which will either cost more or the same as a trip to the vets. If you take them to see a vet then you also get the added bonus of a bit of a general health check. After the buns have been treated their hutch will need a through scrub down and throw away all bedding and hay.