I haven't read all the posts above. Horses nip, but smacking them in the nose isn't the way to go, IMO. Youngsters seem to always "try you" (at least once) by lunging at you - one's size doesn't matter. And, no - it is not acceptable - it is exceedingly dangerous. I have a rule, I will not work around my horses if and when I lose my temper. However, if they lunge at me at liberty (I work a lot at liberty) - I immediatly launched into a pre-scripted "mad as he**" act (which is easy, b/c it does tend to make me pretty angry). I always keep a training whip w me when w a younster and I use it to back up my idle threats while I keep them moving ( I do not make contact w the whip), and I make them keep moving and listening until they look fearfull and/or bewildered (If you are light handed as I am, they get bewildered when mrs. Hyde comes out to play).....aanndd...keep them moving until they look like and are - really regretting it (i.e., sorry). Done. I turn the page - no more yelling, finished. I accept thier apology and go back to quiet sweet "Dr. Jekyll". It is very rare they try it again. What works for a youngster generally works for others.
This this this this this this this.
I really cannot emphasize it enough.
I have an incredibly sensitive young Thoroughbred that we are pretty sure has had someone do something awful to her at some stage [loooong story]... anyway... I don't CARE about her past, if she is rude to me she faces the consequences. The first and only time she ever tried to bite me, I made her think I was going to eat her. I didn't make contact with any part of my body, or with the stick, or with the lead [I always carry a stick when I'm handling horses I'm not 100% sure of], but she sure as heck thought I was going to eat her.
The reaction will be HUGE if you play your cards right, but don't feel bad! I'm not above giving my horse a good hard clobber - even on the face - depending on the horse. I don't hit Magic if I can avoid it, but I have given her more good hard yanks on the lead than I can count, and I've hunted her up in the pasture and the round pen both. My gelding, OTOH, gets whalloped if he so much as looks at me wrong. He is pushier, and can handle it. The force of the hit depends on the severity of the offense of course. A light tap for a sideways look, an all-my-strength-wielding-a-dressage-whip crack on the hindquarter [or neck or shoulder or etc etc] for anything that's actually dangerous.
My horses ALWAYS, the moment I drop my energy and stop delivering consequences, drop their heads and come to me. The low head is key - that is a sign of submission. "You're the boss, I feel safe with you, so you can watch for the lions and tigers, okay?"