I used to help as a volunteer at a therapeutic riding stable in SW Colorado, and one of their best horses had become a biter. He was a grey named Steel and the director was afraid she might have to drop him from the program if she couldn't get the biting under control, as the volunteers were becoming afraid of him. The director, knowing I trained dogs, asked my opinion and advice on the matter. As far as possible I like to use non-confrontational techniques where the animals basically train themselves. After some thought I said I would lead Steel that day. I held a horseshoe nail concealed in my hand which I held next to his head while leading him, in a normal position. About a quarter inch of nail protruded from between my fingers. When Steel swung his head at me, he jabbed himself on the nail, not deep enough to harm him, but enough to feel. He tried it again maybe three or four times, then kept his head forward although he rolled his eyes at me a few times. (He'd bitten other volunteers on the breast hard enough that no one wanted to lead him any more, so we had to put a stop to it.) But we weren't done yet.
When I turned my back to assist the rider by adjusting the stirrup, I was ready for him. This was another of his tricks, he would bite you in the ribs. I had my nail handy and had my arms folded, so he got it in the nostril when he swung his head at my back. He snorted and threw his head up but didn't try it again. But he had one more bad habit.
When you went to loose the girth, he'd grab a big mouthful of your back pocket. That hurt! But again I was ready for him. This time I had the nail in my left hand and my hand in position next to my hip. He swung his head at me HARD - and got the nail on the upper lip right between the nostrils. He actually hit me so hard he knocked the nail out of my hand and I had to scramble to locate it. I walked him around some more to cool him out and he kept his eyes front like a good boy.
About a month later I asked the director about Steel's biting habits, and a look of amazement came over her face. "Oh that's right," she said, "Steel DID used to bite, didn't he?" Apparently he was cured, never knowing when he might smack his face against a sharp point but no longer willing to gamble.