hitting a horse is usually not all that efffective, especially if it's a slap or a "tap". a slap is often just not big enough to make a horse sit up and say, "man, I'll never do THAT again!"
if you must apply some kind of negative reinforcement, it better be memorable and it better make a change in that behavior. otherwise , just as a parent who smack a child regularly usually ends up with an angry, defensive child, you will make your horse more interested in defending itself through bad behavior than making the horse cease that bad behavior.
the other day, while I was out in the pasture I decided to check on Z's feet. he was not on a halter, just standing in the field. as I lifted one of his back feet, he withdrew it and kicked out petulantly, and clipped my ankle. Instantly! I turned and smacked him on the hip with both hands as loudly and hard as I could. it was all I had. had I a whip or a rope in my hand, I'd have really snapped him one. But, all I had was my hands and my voice. But, I did it so fast, so loudly and with so much energy, he leaped away from me then turned to look at me with surprise . Then, I walked over, bent down and picked up his hind foot, and you'd better believe me, he let me do it without a bit of sass.