The List-Unsubscribe header is in every message:
List-Unsubscribe: <http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-talk>,
<mailto:ruby-talk-***@ruby-lang.org?subject=unsubscribe>
In other words, send a message to: ruby-talk-***@ruby-lang.org
with the Subject line as "unsubscribe" (no quotes, nothing else).
Note the "-request" at the end of the message.
This goes for any list running GNU Mailman software.
(disclaimer: I am not an admin, I cannot unsubscribe you)