Saying you have beaten a game is usually reserved for single player games and not for MMOs like World of Warcraft but there are rare instances where MMOs are beaten.
Recently a player by the name of Caspi has beaten World of Warcraft: Wrath of the Lich King. It sounds ridiculous but check out Caspi’s armory. He has completed every achievement in the game giving him 1054/1054 achievements completed. This means he has beaten the Lich King on 25 hard mode, got a Gladiator title in PvP and completed every difficult achievement in the game.
That’s about the closest anyone will get to beating an MMO.