This page is useful:
http://www.python.org/cgi-bin/faqw-mm.py?req=show&file=faq03.003.htp.
The basic approach is:

  ssh to lists.cat.org.au (ie dude) 
  cd /var/lib/mailman

Edit the mbox file.  For public lists, the public folder probably just has a link to the mbox folder in the private folder. 

  sudo vim archives/private/mylist.mbox/mylist.mbox

Note that if you open this file, and then a new post gets sent through
before you save again, you're likely to wipe the new post from the archive.
It would be nice to have a wrapper around the editor which checks for this.
Suggestions?

If you're not familiar with the format of an mbox file, see the link above.

There's some issues though. If you remove a post completely from the mbox,
and then rebuild the archive, the numbering of the html pages for later posts
will be changed. As such it's best to delete the details, but leave a post in
place. You could just leave a stub in there like:

  <blank line>
  From XXXXX@XXXXXX.XXX Wed Sep 26 17:53:02 2001
  To: Mylist <mylist@lists.cat.org.au>
  From: XXXXX@XXXXX.XXX
  Subject: Removed by admin

  Removed
  <blank line>

That can still break the threading connections between articles though,
so it's probably best if you only remove or edit headers that you need to hide.

You then re-build the html rendition of the archive, which is based on the mbox file,
backing the old one up first.

  mv archives/private/mylist archives/private/mylist_bak
  mv archives/public/mylist archives/public/mylist_bak
  /var/lib/mailman/bin/arch mylist

There's still some wierdness though.  If people (often spammers) post stuff
with the wrong date info, messages can get moved to the folder for the date their
message gave in the message headers rather than the folder for the date when it
actually arrived at the mail server.  I don't know any easy solution for that.
For your own machine though, use ntp or ntpdate to keep your system clock correct.