Have only read 1st page. Agree with everyone mostly, esp Tiny & Smilie.
I don't agree with the 'disrespectful' comments though. Horses talk to us in the only way they can, with bodylanguage. If not taught not to, this includes biting & kicking. If a horse feels the need to shout at us. If we're not paying attention to the 'minor' signals, they may escalate. Unless the horse is suddenly frightened or hurt, they don't generally 'shout' without trying to talk to us first. And I believe it is vitally important to listen to & be considerate of your horse, try to understand & 'treat' the *reason* behind the behaviour, not just the 'symptom'.
That is not in the least to say that I would tolerate biting. It will still incur a strong, swift punishment *if I'm in time to fairly do it*, which means at the instant of the behaviour(eg the holding something pointy trick), or at worst, a second later. As others have said, being a dangerous behaviour(even worse than a dog being encouraged/allowed to mouth or play bite a human), it is one of the few things I will punish strongly. If need be. When a horse first tries this sort of behaviour, it may actually take very little in the form of a Bad consequence, especially if you show him you're being considerate of him. Especially if you're also rewarding 'Good' behaviour & make everything Right as easy as possible for the horse.
Unfortunately, if the horse has a history of doing this, especially if there's been inconsistent & ineffective punishment, in order for it to be effective(which it should be in 1 - few times, or you're missing something), it tends to need to be quite extreme. The horse may think of it as a game, or challenge, of 'you hurt me but I can hurt you more' or 'I'm quicker than you'. So they need to get the message, in no uncertain terms, that this is no game & it is Very Bad News for them if they bite a human. *In all my years dealing with horses, I've only come across 2 that I felt needed to go that far tho.
I believe the biggest key though, for a horse with a great 'work ethic', great manners & training is to focus on & reward the Good Things you get, and to a large degree, you can then ignore the 'Wrong', the mistakes they make. (not biting tho, don't ignore that!) While I don't use a clicker & tend to do things my own way, don't always use food for eg, I basically use Tiny's kind of method - 'clicker training'. If you don't know about it, I advise you look into it. Pay particular attention to the *principles* behind it. If as well as reinforcing other 'manners' & training, you also make a point of teaching/rewarding some behaviour that is contrary to biting - say your horse tucking his nose in & away from you, there's one more good reason for them not to bite, because they can't do that while getting rewarded for keeping their nose to themselves. **Also very important, regardless of what your horse may have just done well too, ensure they NEVER get rewarded