How to restrict access to a non-drupal url on a site's domain?
Nope. It sounds like your web server has the mailman web interface alias to that path. If that's the case, Drupal is never evoked so it wouldn't have any say on how that request is handled (i.e. index.php?q=mailmain/listinfo isn't executed for page handling). 

You'll need look at configuring your web server to remove your alias, or remove it for your hostname.