[Mailman-Users] non-web unsubscribe

Is there a way to remove a user from a list without logging onto the web-interface? On our site, the web-server is limited to intranet users only, but the list is sent out to people all over the world. I want to automate their removal, but I have tried sending an email saying 'unsubscribe [email-address]' and nothing happened.

Thanks, Duncan McQueen