I use repeal-x spray. I also have flytraps in the barn, and use fly predators on the manure pile. Just added the fly predators this year and I think there is improvement. I also spray the stalls in the morning before they come in. The fans help in the stalls, too.
We do get horseflies and ticks really bad here and nothing works on them. I mix the repeal-x strong and that helps a bit. Also, almost all fly sprays say that they will work for 3 or more days. I spray the horses about every other day.
I really hate flies and so do my horses.