Well there are plenty of sprays you can get but what I do with my horse is to have him "fly poured".
This is actually meant for cattle but works just as well for horses, it is a liquid that is poured from the poll to tail and as long as the horse isn't washed or brushed in the next 24 or so hours then it will keep the flies away for several weeks!
My yard manager actually fly pours my horse when she does the cattle so I would recommend getting someone who knows what their doing to do it and to just check that the type of fly pour they're using is suitable for horses too.
If you are in any doubt though, or your horse has sensitive skin or allergys I would definitely go with a spray manufactured and designed for horses, just in case and check up with a vet beforehand, better safe than sorry!
I use anything from Zonk It, Tritech, or Endure on my horses; it usually winds up being the ones on sale out of those...and I have used these on their faces with no issues...I usually will wipe it on the faces, near the eyes, rather than spray, though, just in case.
I also feed Apple Cider Vinegar through out spring-fall, and that helps ALOT especially with the little black stable flies...nasty little biters that they are!