Cowboydad,

Their are numerous things to try.
*Apple cider vinager, if your horses will drink the water that is great, wouldn't want them to become dehydrated, or you could put it on their feed. I want to say it is more than a tbls if you put it in their water.
*Garlic works also just like it does for people you would have to see if your horse would eat it. You know the garlic is working if you can smell it on their breath. You have to be able to smell it. There are a number of commerical garlic products.
* There are the fly preditors.
* There is the feed through fly control which works well also, my neighbor uses it although it is expensive, and can't be used on pregnant mares. she has noticed a considerable difference in flys.
* I had a client tell me the other day that she uses a topical wipe that she applies once a week made by pyrannah. and she has no flys.

I think it is a matter of what fits your budget and what works for you. Good luck on controling those nasty little bugs.