[Mailman-Users] Can I prevent all members, except list manager, from sending mail to the list??

[Mike Hogsett]

Alternatively...

replace the "|mailman post <LIST>" alias with a

LIST: /dev/null

or other destination (which does not go to mailman post)

and then create an unadvertised aliases such as 

LIST-really-send: "|mailman post <LIST>"

perhaps that will do what you want.