RULE #1
Whether you have a playful biter or an aggressive biter, you need to implement the correct tools for success. A 4 knotted rope training halter and a 3-4 foot training stick or crop will give you the necessary tools to correct the behavior more easily and effectively while also keeping yourself safe.
In order to fully eliminate a bad biting habit, you will need to grow accustomed to not petting your horse on the head while handling and working with them, and refrain from giving them any hand fed treats. If you want to reward your horse for a working task, pet them on the neck only during handling and training. This will be the first step to establishing a healthy boundary.
Horses must learn that your hand isn’t for feeding or petting, it is their primary guide and body language in their training that is owed respect.
HOW TO FIX THE HABIT
Do not hold your lead rope with too much slack. Commonly, handlers hold their ropes with 1-2 feet of lead line slack when casually spending time/leading their horses, and this allows the horse to push their faces (and bodies) directly into your personal space. By shortening the lead rope and holding it on or right below the lead clasp, you will have the proper leverage to bump the lead rope and make full and complete contact with the 4 knots and chin knot on the training halter, so the horse feels their cue and correction to the maximum.
If the horse leans into your space and/or tries to nibble at your hands, immediately use the lead clasp itself to firmly bump the horse’s nose away from you. Or hold the lead line on the clasp, make a fist with your hand and bump the horse’s muzzle area or side knot of the training halter. The key is to do this instantaneously to the horse pushing or biting. When you see the warning signs (the horse starting to lean into your space), immediately correct him, even if the biting hasn’t occurred yet. By staying proactive, the horse will understand this boundary faster.