Those are called "trolls." Their attacks that attempt to disrupt boards are called bombs. They will also assume others' identities (by creating a new account and changing the name slightly). Most message board engines today are much more robust and resistant to damage from these hackers than they used to be; however, these creeps can still obviously be disruptive and cause the administrators trouble and keep them busy deleting posts and banning members.

Their attacks have caused problems on many boards - they caused Rolling Stone magazine to stop hosting a message board altogether. I hope it's just an aberration here. My advice - nip it, nip it in the bud. Ban them fast, every way you can (by email, name, and IP number).