[MM3-users] Sorry to repeat a question, but how do I permanently delete messages, again?

I askled this question some months ago, but for the life of me cannot find the response (though there is one).   I want to purge some specific emails from the list archives.  A user posted what he thought was a personal message to the list, and is embarrased by it, so I told him I couldn't delete emails that were already downloaded onto client machines, but I can delete it so that new users won't be able to see it.

But...   I can't figure out how to do it.

I asked this once before, some months ago, and as I remember there is a command line way of doing it, but not a GUI way.  But i can't remember what it was.

Thanks!

billo