The spot-ons are just concentrated versions of the sprays you use, and they work best for about a day. They do help a lot in heavy tick infested areas, for the alloted time, but flies are just persistent little buggers!
I think your best bet is to not only use your normal spray (apply when the flies are the worst, and then before you put the horses away, apply a non-toxic, repellant. It may not last long, but it's a short reprive and not overloading your horse with toxic chemicals (or yourself). Fly sheets and masks do help. As do box fans in the stall (flies don't navigate well in high winds-one of the few advantages to KS wind I"ve found. LOL)
You have read the thread on feeding ACV, I'm sure, but I have no idea if it would work for you, but worth a try. Making sure your facilities are immaculatly clean helps, so do fly predators, but using fly spray kills the predators! There's feed thru fly control (contains rabon) and it works, but can be expensive if you have several horses, or if you board, it's a waste as not everyone will use it, so their horse's poo still breeds flies. Spreading/gathering poo in the pasture helps (composting kills fly and worm larvae).
Fly strips and any bait trap does help. Every fly caught is one less able to reproduce, and it's morbidly satisfying to see the bodies build up! LOL I like the old fashioned jug traps that have water in the bottom and the flies just can't figure out how to come back out and drown.
You just have to empty once in a while. There are disposable versions, but I like to reuse/recycle things, so I dump the traps.
Other than that, I'd say flies are just a fact of life for horses, especially if you have a neighbor with a barn lot full of cows across the fence (*sigh*..that's MY problem-almost a feed lot with NO manure clean up and my boys get eaten alive, but we are fixing to move....)