Blizzard posted an official annoucement on their forums that they have kicked over 320,000 accounts from Battle.net they found were violating the terms of service.
We would like all players to remember that abuse of unintended mechanics and/or use of third party programs is a violation of the agreement made when signing on to Battle.net, and can subject your account to disciplinary action up to and including a permanent ban of its access to the service.
Many account closures come as the direct result of tips emailed to our hacks team by legitimate Battle.net users.
This is not the first time Blizzard has done something like this. In November of 2008 they banned over 350,000 accounts from Battle.net stating, "Cheating ruins the game experience for legitimate players, and we will not tolerate it."