Honestly I find it appalling and a serious overstepping of your bounds as game mods that your response to someone reporting a non-game-related argument on plurk was to give the other party a warning instead of just advising the person with the complaint to defriend them and come back to you if the problem persisted.

You are game mods. Your jurisdiction has boundaries. If you expect your players to respect each others' boundaries, you also need to respect your players by respecting the boundaries of your own authority.