I have to agree with RiosDad. In my experience, 90% of people who claim hitting doesn't help is because the person has never actually HIT a horse before.
When a horse bites or kicks, there is NO foolin'. You have two options - you can go the natural horsemanship method, or you can let fly with your fist firmly into his nose. This isn't a "Bad Bobo" smack, this is "Horse, I'ma knock your bleepin lights out if you try that again!" backhand.
I have never had a horse get past one or two of these and continue with thinking they want to be biting me. I have NEVER had a horse become headshy - like RiosDad said - they become headshy from "swats" constantly. Nail the sucker across the chops just once or twice, enough to make stagger a step back, and I promise you will have a non-biting horse that is not headshy. They KNOW. If they went to bite the leadmare, they'd get a rock solid hoof with more power then a semi doing a buck twenty down the highway right to the temple.
I don't believe in "hitting" a horse. Swatting him on the nose, or slapping him on the chest is only going to piss him off. If you ARE going to use physical discipline, it needs to be as fast and furious as you can bring it down on his poor head and over before he even knows what hit him.
If done right, I have never in my life witnessed a horse act afraid, headshy or abused after getting a licking for biting, kicking, or any other nasty dangerous vice.