Get rid of your horse?
I wish I had a good suggestion, since We don't have them here.
I hate ANY biting insect, be them Mosquito, horsefly, deerfly, gnat, greenhead fly or "noseeums". On top of the nuisance of the biting pain and itch, there's also the possibility of viruses that can be transmitted. I got a nice case of ringworm years ago from a jumbo horsefly. I killed the sucks, but not until after it delivered it's "gift" in my leg.
Bug spray works for some, but I can't speak for the larger biters.
Short of wearing long sleeve shirts and long pants, there may not be much you can do as long as they are in their feeding cycle.
Good luck!