You could try to get them at the source. Flies need moisture to reproduce; specifically moist rotting organic matter. Make sure your horse poop piles are at the very least spread out so that it can dry out before the fly eggs can hatch. (here in summer, the spread out piles are dust within 8 hours) Also make sure any old wet hay (or any other moist rotting organic matter) is removed, spread out to dry or burned.
The other thing you can do is try a feed through fly control. It is a pellet you feed the horses that has a chemical in it that goes through into the poop and keeps the fly eggs from developing. Personally, I don't like putting pesticides into my horses, but you do something similar each time you worm, so it is up to you how bad of a problem it is and how far you want to take it.