@ihnorton do you know where the landing page template is configured? I already changed the introductory description but I don’t see a place to turn off signup links.
From a quick search it seems that it we could disable the list using “emergency” moderation mode [1] together with an autoresponder message [2] that explains the move to discourse. If there are no better suggestions I can try doing that (or pass over admin rights to anyone interested in experimenting).
http://www.gnu.org/software/mailman/mailman-admin/node14.html