Wbm in the game Runescape stands for Wanna Be Mod which refers to a player who is acting as if he has the power to ban people. Players act this way because Jagex (the makers of the game) promote well behaved players as Moderators.
"Omg stop telling him which thread to post in you WBM"
"Omg another WBM invading my thread"
"Stop reporting every person you see you dumb WBM"
