I speak for Braehead only here and I'll keep it short as there as not much to say. Problems on slope are actively dealt with. It's in nobodys interest to have unsafe people on the slope. People are given a chance when they are spotted (advised to try from half way) if they say they have not been on skis/board for a while etc, it would be rather harsh to have them leave after one run unless they are blatantly not even able to balance upright on a pair of skis or board.