I would put stopping doing something to someone when they tell you to stop pretty high up on the list of Great Moral Rules of the Universe.
Basically, the thing that separates martial arts training from physical assault is the consensual nature of it. Take away the consent and you're assaulting someone, and whether it's 'for their own good' or not doesn't change that. Suggest to them that they should push themselves harder, tell them you don't think they're at their limit, if you're in charge of promoting them then don't promote them, heck, don't let them train if you feel strongly, whatever. But ultimately it's up to them and it doesn't matter if you're O-Sensei himself -- someone tells you to stop, you stop.
On those rare occasions when someone hasn't respected my tap, my first instinct has been to get off the ground as fast as I can (once they let go) and punch them in the face. I haven't actually done it, but is that really the kind of energy you want in someone who's going to be torquing on *your* arm in a few minutes?
I'm lending you my body for your practice. As with any borrowed property, treating it as if it were your own is the *minimum* standard for behavior.