Having played on Minecraft servers, I can wholeheartedly declare that people use the n-word out of casual annoyance, and that the people who take stuff said anonymously online seriously are pitiful. Having played Call of Duty I can wholeheartedly declare the n-word used commonly as an attack. Having played WoW I can say the same thing. Maybe you're in for a rude awakening, but shit said in PMs should not be punishable unless it's an attack on the person being discussed. If you want a world like that, where private conversations are not protected, idk what to say. The world is not your safe space.
Going full "no tolerance, no forgiveness" towards these people is naive as fuck because I'm willing to bet you yourself have even said that word. if you take that approach towards anyone who says something mean, you're going to find yourself playing alone very quickly.