A horse can't tell the difference between a nuzzle, a poke, a 'little nip' and a 'real bite that hurts'. So I ithink the best thing is to get after them for any nosing or nibbling. Hopefully, someone else has already worked with your horse before you get it, and you don't have to stop them from biting. But if you raise your own you see JUST how they come out of the womb, which is for some of them, that people are sort of a fun chew toy!
I know a lot of people think that nosing and poking is cute and a sign of affection. I don't think it is in the horse's mind. For the horse, I think it's exploration, to see if some rough play is going to be ok. That's what they do with each other. First a little nosing and exploration, and then yay, BITE BITE BITE! Humans must be very, very strange critters, they just don't play that way! They give all the signals that they want to play, then you play, and for some odd reason they start screaming and crying 'ow! ow! my haaannd! you bit my finger off!', can you imagine! Humans are so hard to understand!
My friend got a horse from a lady that never let anyone get after her studs if they bit. One of her studs was very nice when she got him, but after a few years he was so bad, he put someone in the hospital. It's better to just swallow hard, forget about being 'baby's mommy' for a little while, and make sure they know not to bite.
I do have one horse that has never 'escalated' from nosing and nibbling to biting, and others may have a horse like that too. Lucky you, that is actually very, very weird and unusual, LOL. I'd say most horses just are not like that. I've got that one, and I've got two others that are very, very unlike that. You let them nuzzle, the next thing they are biting.
I don't think that most slapping on the neck or shoulder is seen as discipline, unless the horse has been made very, very afraid of people by rough handling, and thinks that after that slap the sky opens up and all heck breaks loose.
With a normal horse, it's just how horses play with each other. I once watched a gal slap a 2 yr old stud colt for about an hour and a half, LOL. She'd slap him on the muzzle, he'd bite, and she'd slap again. He had this look on his face like, 'THIS IS FUN!' I said, 'he's biting you', she said, 'i know'. I said why don't you do something different. She said, 'i don't wannaa HURT him!' I said well, if he winds up in a can of Biljac because he bites, that would hurt him more. She shanked him. And he seemed to say, 'oh, well, fine, why didn't you say so in the first place', LOL.
I don't much like to hit them around the mouth or face with my hand, either. Logic tells us it's going to make them jerk their head back when you try to bridle them or give them medication. Sometimes you just have to react fast and that's what happens. So sometimes you just do what you have to do, if it works, so be it.
So normally what I do is put a chain shank over the nose (wrapped round the nose piece of the halter so it's not dangling and loose, which can be dangerous, and wrapping it also makes it not as harsh). When they come at you to bite, jerk, every single time. It is uncomfortable and unpleasant, but not so much it makes the horse rear or run back. I don't want to shank them really hard and make them run back or rear. I want to keep them with me and thinking about it. If they really do land one on you, you have no choice really but to be very, very dramatic, like the gal above described. Make it memorable.
When they bite or nibble or nuzzle, I yank on the shank, fairly hard.
