1st time apply the aid lightly, no response? 2nd time apply it more forcefully, nothing? 3rd is the strongest, briefly of course (half-half lasts a nano second only), give it all you got! Nothing still? Stop & back the horse up. Do this everytime he ignores your half halt. I stand by the famous quote "A good rider rides from transition to transition. A great rider rides from half-halt to half-halt."
Thanks very much for the advice - I need to think on this...not to think "are you right" but to think about how this will work with this guy and what he's doing.
I half-halted and he took no discernible notice of this at all. The instructor has told me that you *can* hold a half-halt up to 3 seconds, so I tried that. It was amazing. He just kept right on running through his trot, only with his head a lot higher up in the air. So then I really sat firmly on him, let my weight start dropping firmly into the saddle, not lightly, firmed up my core, firmed up my arms, took up the reins some more, and did the equivalent of slamming on the brakes.
Even with that, it was another quarter-trip around the ring before he slowed his trot and stopped running through it...and when I let up on the pressure, he went right back to running through the trot.
I have learned to back the horse up from the saddle, but I haven't spent a lot of time practicing this. This guy definitely does not like to back up, so I use that a fair amount when he gets pushy and obnoxious on the ground.
I wonder if we just need to go back to the drawing board on this guy and back his butt up every time he ignores one of the aids...what I suspect is that he's got some basic problems with disrespect (which he does to everyone), and on top of that, the horse camp experience has messed up his responses to the aids (I'm assuming it's that, because this is when his behavior started to really go downhill). Seems to me the backing-up approach might work to resolve both of those issues...