The "closed"  letter isn't  a letter,  but a command  reply. That  is, it
isn't sent as a separate  message like the other administrative messages,
instead it looks like this:
 
> subscribe xyz-l Joe User
Sorry, the XYZ-L list is closed. Contact the list owner for more
information.
 
You can't configure command replies, only the letters. But you can indeed
set  the  list  to  By  owner,  and configure  the  letter  sent  to  new
subscribers. You can set  it so that it only goes to the  user and not to
the owner. I do that for FISH-ECOLOGY.
 
  Eric