I use permethrin. It's good for a lot of different pests including flies. I have a lot of coops so when I treat one I treat them all so I buy the concentrate but for people with one coop you can buy the premixed spray. I also put poultry dust in the nest boxes. Initially I was spraying my coops because I had found mites in a couple of them and it was fly season so there were a lot of flies in the coops too. I was amazed. I sprayed the coops weekly for a month to get rid of the mites but it also eliminated the flies. It was so nice to walk into a coop without being bombarded with flies. Good luck...