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Seems like Grincher can corrupt save data. So, do not open any mail from Grincher, leave any room you see Grincher in, and do not join any room with a 1,000 player limit called "Grincher's Happy Room". This is not a joke.
Edit: Kicking Grincher from your game does not work, they are not affected by the kick action.
It's really freaky to experience it in-game. They keep joining my games, and they can't be kicked, so they just sit there, menacingly. As soon as I found out they could corrupt save data I just started leaving whenever they joined. It may or may not be true but I don't want to risk it.