Thanks mate
sorry for delay in response
simply put: don't put up with it - injuring students is not part of the process of the student learning, unless you are learning how to be injured...?
To me, the way I see it you offer someone your body to demonstrate a technique on, in good faith and with polite compliance they go too far and strain a tendon in your wrist or whatever then they are what I would call an "asshole" and I don't train with "assholes".
If you are going to an MMA/ boxing/ kickboxing class and sparring then you can expect to get hurt, but not injured.
Though the longer you go and the harder you go a little injury here or there would eventually become inevitable - but this is a world away from overzealous "demonstrations" - what exactly do these demonstrations prove?
Thats its easy to hurt someone if they let you?
Well... durr.