http://wowwiki.wikia.com/Griefing"Griefing refers to the act of a player ruining the gaming experience of another player on purpose (though this is debatable from one server to another - see below). The most obvious form of griefing is constant stalking of a player, by either following him or her for no reason, whispering useless messages, repeatedly sending them invitations after they decline, etc."
"A griefer in World of Warcraft is almost always one who PKs or trains mobs to other characters, but the term
includes any activity specifically designed to hamper another player's gameplay, progress, or fun.Griefing can also be harassment as it is defined in the Blizzard's Harassment Policy, which can result in disciplinary action for a player."
It's likely you won't find a picture perfect quote of someone complaining they got banned for a very specific instance of "griefing", since the term "griefing" encompasses many different things and is most likely determined on a case by case basis, as was this one. Blizzard "disciplined" 5,400 accounts because of "griefing". That could be anything, including the intentional disruption of player quests hubs for hours/days on end, because, as stated; "any activity specifically designed to hamper another player's gameplay, progress, or fun.".
It's pretty solid evidence to me. But then again, my name ain't OGTucker and I'm not retarded.