Most of the horse fly sprays double as barn sprays, just read the back for instructions.
However, I would recommend Repelx and/or DuraGuard. DuraGuard is, of course, more expensive and, luckily, Repelx works the best for us, kills them in the air.
One thing that is also really working for us (don't put it in the stall) are the fly abatement traps. The bait is contained within a little plastic strip and the bugs drop like flies.
If you use the strip, be sure to put a container under it to catch the thousands of dead flies.
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Which one are you using? Tired of spending my money on stuff that doesn't work so well!