I think this is a great idea because as the game progresses, the powerup spawn rate increases as a means for survivors to counteract the greater number of snowmen. If the number of snowmen does not increase because people leave when they get converted, the survivors are simply too powerful. Snowmen will end up losing if they do not convert survivors fast enough, but will also lose if they cannot gain teammates. Disallowing /hub, /lobby, and similar commands would not work because people who want to leave can just exit the server. I think loss of medals could work, especially for competitive players, because they determine the leaderboard. Leaving should cause a loss of all the medals you earned in the game you just played. Continued instances of leaving games early should cause a loss of all the medals just earned plus more from previous games. Further continuation should cause a temporary ban from Snowman Survival (for the less competitive players).