I was working in the woods planting flowering trees like hawthorn, redbud, dogwood, crabapple and crape myrtle because the deer have browsed out almost all of the understory. The deer flies were annoying.
I put on some DEET although I've never had much success with it against mosquitoes, midges and that sort but I am here to endorse it against deer flies. I could hear the little buggers coming for my ears like Luke Skywalker attacking the Death Star but DEET worked like a force shield on the flies. When they got within a few inches they skittered off.