There really isn't much within LISTSERV itself.  You need to bring in a genuine spam-filter technology of some kind.

If you have a 15.x version of LISTSERV running, you can enable the SMTPL 'listener' spam exit and feed mail through SpamAssassin (or other such engine you may happen to have) for scoring and pass/reject based on that - the method for implementing this is in the documentation and it works (I've done it on my site).

Alternatively put a filter appliance (there are many out there, some commercial some open source) into your LISTSERV incoming mail path; your organization might already have one in the general email system configuration, ask about using it.

>>> "Sakr, Tamer" <[log in to unmask]> 03/20/09 12:30 PM >>>
Hello,

We currently experiencing overflow of spam messages from non-subscribed
addresses sent to various lists.  We see quite a bit of those
transactions and invalid confirm messages to list moderators in
listserv. we have been informing users not to reply to these bogus posts
(which contain typical spam messages).  Regardless some spam will slip
past any filters. I am trying to avoid making changes to listserv to
reduce backscatter for those messages by suppressing some of the
administrative replies that listserv sends because it will make it
impossible for legitimate senders to confirm their commands, postings,
and subscription requests.

Is there a way to configure listserv to ignore posts from
non-subscribers, rather than to send reject/confirm messages?

Regards,
Tamer