This happened to me when I was about 14 in karate. I was big for my age, but still a kid. This dude was, well kicking the crap out of me during kumite, he was about 30 or so. This was due to his perception that I wasn't tough enough or something along those lines. Sensei warned him once during class; that he must take into consideration my age/ experience etc and try to help me learn, not beat it into me. He ignored this and Sensei angrily stopped our sparring. Sensei sent me off to work with someone else, then proceeded to beat the snot out of the dude.
Although I'm not sure if this is the correct action, it had the intended outcome. Dude learned his lesson, and Sensei took the responsibility to teach it to him as I was obviously not able to do so.
I would say that if your Sensei allows this situation to continue that the students must do whatever is necessary to ensure their own safety. That action could be actively petition Sensei to kick the guy out.