A: Many of us deal with horses who want to be mouthy. One of the first things I always check for is dental problems. I once had a horse who was extremely nippy, and when we had him examined by the veterinarian, we discovered he had a horrible situation in his mouth. When we resolved his dental issues, the nippiness subsided.
Once you have ruled out veterinary problems, you can consider this issue from a behavioral point of view. You are right---you don't want to let your gelding continue with this mouthiness because it will likely turn into biting over time as his respect for you diminishes.
The first thing you must do is establish your personal space. Imagine a bubble around your body. Do not let your horse enter this bubble unless you invite him in. My rule is that if a horse is in a disrespectful frame of mind, I do not allow him into my space. If he tries to push into my space, I use my hand on the lead and, with a straight arm, push him back out. You can also use a stick to drive him back by waving at or tapping his chest. Voice cues don't work as well for gaining a horse's respect.
If your horse gets into an especially disrespectful frame of mind where he insists on chewing on everything, then you have to take additional measures beyond establishing your personal space: Move him around. Get his feet working and redirect his thinking to other things. Once he is calmly doing what you ask, then bring him back in close.
Mouthy horses will often chew on their lead ropes. Instead of pulling the rope back out when your horse grabs it, I would suggest pushing the lead further into his mouth and wiggling it back and forth. The intent isn't to hurt him but to create enough discomfort that he wants to spit it out. The goal of this technique is to change his thinking from "I want to grab that" to "Get that thing out of my mouth!" Once he starts wanting to get strange objects out of his mouth, he will start to think twice about whether he really wants them in there in the first place.
I hope this helps you. If you keep your horse's respect high and redirect his energy when He's in a frame of mind where he wants to grab and chew on everything, his behavior should improve over time, and he will be calm and ready to do the things you want or need him to do.
Natural horsemanship trainer and clinician
Abbotsford, British Columbia