The process is outlined [here][1] by [Jeff][2]. Basically if someone is constantly being flagged as offensive or spam by other users, they are sent an email by a moderator. If the behavior does not change, the user is suspended. This is only done in extremely rare cases (I've only seen it once - not that I actively look for it though)

It is not done anonymously, it is done by a mod and there is notification given via private email to the offending user.


  [1]: http://blog.stackoverflow.com/2009/04/a-day-in-the-penalty-box/
  [2]: http://meta.serverfault.com/users/1/jeff-atwood