Blizzard has banned "several thousand" Diablo III players for using bots, according to this Battle.net forum thread. In announcing the bans, Blizzard said that these kinds of acts (using cheats and exploits) are "undermining the spirit of fair play," and that the company plans to release new tools in the early part of 2013 that make it easier for players to report bad player behavior and cheating to Blizzard.
Blizzard community manager Lylirra announced the bans on the Battle.net forums:
"We’ve recently issued account bans to several thousand Diablo III players who were found to be using botting programs while playing. In addition to undermining the spirit of fair play, botting, hacking, and other forms of cheating can also cause technical problems with the game as well as contribute to performance issues with the Battle.net service.
As always, maintaining a stable, safe, and enjoyable online environment for legitimate players is very important to us, and we’ll be continuing to keep watch on Battle.net and take action as needed.
We will also be adding the ability for players to report this kind of behavior directly through the game client in a future patch."
The statement from Blizzard goes on to encourage players to report other players they suspect of cheating, using bots, or taking advantage of exploits.