This occured awhile back: I had read that vanilla Christmas tree car deodorizers which are hung from the rear view mirror in a car deters flies in chicken pens/coops. I went and bought a pack at our local auto parts store and hung them around the coop. Sure enough they seemed to work well. You can give it a try if you wish.
I may try some of those. Good for the car too. I'm not a big fan of vanilla though, but like the green apple and blackberry clove. Perhaps something similar with citronella would also be effective. The coops never get too many flies since I pick up under roosts each morning. It is outside, in those shady areas during the Summer, where they seem to loiter.