I agree with the lot of you. It is an act of dominance. And like someone said, if this gelding did it to a dominant horse, that gelding would probably get more than a slap or tap. I honestly wouldnt feed treats from your hands either. Be it grass, or whatever until the nipping gets resolved. Elbows work wonders. Or if that doesnt work,,, the crop. You don't neccessarily have to pop him in the face or mouth, but one good pop on the butt will make him think twice. If he goes for a revenge bite,,,take him and show him who's boss. Move his feet however wherever.