I have heard of people using garlic. They say it helps, doesn't solve the problem, but helps. But do be careful as garlic can make a horse anemic.
I'm using Apple Cider Vinegar. I don't over use it, I put 2 cups in 30 gal. Of water. She loves the taste. It takes about 4 to 6 weeks for it to do much good, but I am now noticing a definite drop in the flies, and bugs hanging around her, and very few on her.
It also helps with digestion, and helps to prevent bladder stones (we have super hard water here.)