How about changing the address of the list so that any references to the old
list would be bounced.

The problem with a "subscribers only" list is that I have several aliases
used to deliver mail to my mail box, including one for team-ada. I use these
as anti-spam tools (see address of this email) so that I can give a valid
address to locations on the web (or as a reply-to address) that need them
but I never send mail from these alias accounts. If that address starts
getting spam I can just remove the alias and I'm done with it. Some email
clients allow multiple "personalities" which works well with this, but not
all do.

Robin Reagan