No sprays (and I've tried all kinds that are available here) seem to work for more than 5 minutes on our monster horseflies, so I decided just not to spray him with those chemicals - they don't help anyway. We live by a swamp and a river, so the amount of bugs is natural, just annoying - all horses in our barn get a vacation during July. If I decide to ride, I put a fly mask and a riding flysheet on him, those protect him at least during the ride, but I can't leave them on when I let him out in the pastures, because his herdmates are quick to undress him. :)
There's really nothing I can do, and at least he is comfortable during the night, when the horseflies sleep and the horses leave the shelters and graze in the pastures. So I'm having a wonderful cantaloupe right now, just to comfort myself. :)
I have come a long way, to surrender my shadow to the shadow of my horse.