Pyranah Spray N Wipe (or something like that) works really well but it is not good to put a lot of it on your horse.
Have you tried skin so soft? That apparently works too.
If all else fails, fly mask and sheet or at least put some SWAT on the underside of their barrel from between their legs all the way to the umbilical cord scar (or just before their privates.)
This is just an idea (so PLEASE research it or something..) but those OFF things repel mosquitoes and flies.. maybe see if it's safe to attach one close to the barn or water bucket or something to keep them away?
"Strength is the ability to use a muscle without tension"