How are the school dealing with this behaviour? If hes punished there, then I wouldn't punish him further at home, but I think some serious chats about respect for other peoples space and privacy are in order. By saying that I'm not implying you haven't done this already, but clearly he needs some hefty repetition for it to sink home.
You could ask him how he would feel if someone did that to him, in front of everyone. And let him know that the girl may well have gone home and cried about it, and she might start being scared to go to school etc, from something like that happening. If he sees his actions as having bad consequences for the other person, rather than just a big joke, he might think twice next time.
As a last resort, (I'm only saying this as something I might do in your situation, mind you) I might be inclined to threaten him that if he did anything like that again, I would be getting the Heads permission to come to school to sit in lessons and watch him, and that his friends would think he was ridiculous for having Mummy come in to school to make sure hes a good boy. I think the fear of being embarrassed could deter a boy from repeating the behaviour!
