Sprinkle food-grade Diatomaceous Earth around the feeders and in the coop and nesting boxes. I also add some to my layer feed. Flies are not supposed to be attracted to the feces of animals who eat DE.
CHICKEN CRACK BOTTLES Use a 1/2" drill bit to drill holes in empty plastic bottles, add chicken scratch (aka: chicken crack) and watch the fun break out! Provide several bottles to the flock at once to avoid conflict & fowl penalties.