Horses are playful creatures by nature. They indulge in rough play with other horses, which often times includes head and body banging and biting. Therefore, when your horse starts biting you it may mean that they find you to be their equal, and that they want you to play with them.
There are hierarchies in herds of horses. The dominant horse in the herd is known as the alpha horse. Other horses in the herd never bite the alpha horse. Therefore, biting you may also mean that they do not find you to be dominating.
Other possible reasons for biting may be that there is something that is hurting them such as a pinching girth or a prickle under their saddle pad. So before you discipline your horse, it is important to assess the situation and find out what may cause this habit before addressing it.
Once you have a fairly good idea of what the root cause of the biting may be, and you determine it is a behavioral issue and not a physical issue, then there are a few things you can do to eliminate or lessen this bad habit.
Sadly, many horse caretakers want to punish the horse in a way that causes even more harm. Many horse owners would smack or hit their horse. This is not a healthy exercise and may cause your horse to become disrespectful and mistrustful. This will not aid you in your training. Patience with your horse is vitally important at this stage.
When you have established that it is a behavior issue, then communicating directly with horse will allow you to quickly get to the cause of the biting and help you establish a more respectful relationship. Only then can you get back to the business of performance training. A horse won't readily take your lead unless they respect and trust you.
Another good way to keep your horse from biting you is by teaching your horse to move around you and have them follow you instead of the other way around. Horses need to answer to an alpha, otherwise, they will feel the need to assume the role of the alpha.
Also, you can help your horse by playing games that will help them use their energy more positively. Games that include oral interaction such as teaching him to pick up objects are good ways of managing the habit. By playing games, you can also interact with your horse and show him that there is fun without having hurtful "horseplay".