I affirm the effort, but prioritize safety.
I have a fantastic student who pushes 100% all the time. Being a martial arts instructor, there is inherently more risk in some of the exercises we do, and there have been a few instances where I have had to say, “Your done. It is not safe for you to continue.”
She is always disappointed initially, but later appreciates me more for caring, and looking out for her safety. A gentle, honest word can go a long way to change attitudes.
Your redirection to overarching goals will help them from focusing on overdoing a particular exercise.