I don't think this is true. This was is Christopher Poole's testimony:

  Q. Now, you said that 4chan does not keep the
  contents of these posts. Does 4chan maintain business
  records of web logs, also audit logs and sometimes banned
  entries as well, if a particular user has been banned?
  A. Yes.

http://www.scribd.com/doc/41691202/35688046-Christopher-Moot...