My horse in mean to other horses in the ring!?

When I got my horse he was a loving TB who didn't care much about anything but now he is nasty to the other horses in the ring. My instructor told me to reprimand him with the crop or my hand when he does that but that makes him kick out or get feisty I don't want to endanger other horses so I try to keep him by himself but this is a new problem and i don't want it to escalate...what can I do to stop my horse from pinning his ears and possibly kicking another horse?