timzsta- except there are no policemen for this role.
So it leaves the problem, refuse to train them, whereupon they go elsewhere and possibly encounter a less experienced instructor who is caught out by one event too many.
Or do you train them to the best of your ability, try and teach them the correct way to do things and how to stay out of trouble (if they accept your advice) and then hope something sinks in and they don't kill anybody else if and when it happens.
I've tried both- one's still flying (I generally avoided flying with him) and has not hurt himself or anyone else (yet) but continues to gain the attention of the authorities and scares others regularily.
Another tried to self teach himself strip skills- only lost the nose spat. I finished his training but realised after the fact that if he saw something being done he would consider himself to have been "taught" and would try it.