I had the problem were my gelding will dance ALL around the post I have him tied to. Theres just no grooming him. I start with a firm 'Loki!' and if he doesn't quit dancing around and fidgeting, I give him a firm 'LOKI!' and a smack on the shoulder with a riding crop. I very, very, rarely use my palm. Yes, he will try doing it again, but with a firm, consistent 'No!' command to the impatience, he should soon quit.
Don't really lay it on him, and don't hit him anywhere near the face, of course. I find that a 'reminder' on the shoulder or hindquarter works wonders. I spent a good two-three hours untangling demon knots from his mane with barely scoot or a 'I'm bored!' cue.
He actually ground tied out in the pasture the other day, something he's never been taught to do. If he started to move, I would give him that firm 'Loki' warning tone (no smack with a crop) and he stopped, relaxed, and slept while being groomed.
Loki actually used to untie myself, pull his head down, move from side to side, back and forth, circles. It was a nightmares doing ANY grooming. He didn't respond to vocal commands and I just irritated and frustrated. After that, when he moved, he got a wack. Moved again? Same thing. Now? No more moving around 8D
Wait! I'll fix it....