You could implement the SPAM_EXIT feature, documented in the Site or Advanced manual (don't remember which at the moment), and in your local script detect and block items claiming to be FROM: your own system.

The CONTENT_FILTER mail template might also be useful in this context, but I haven't tried it for this particular problem. 

>>> Peter DiCamillo <[log in to unmask]> 10/01/09 2:43 PM >>>
We have a list, THURSDAY-SUPPER, which has "Send= Editor, Hold", and a 
number of editors and moderators.  Spam mail was submitted to the list 
with a forged From address which was the list address.  (Both From and 
To addresses were the list address.)  Listserv requested confirmation 
from the moderators as usual.  However, the moderation response was 
posted to the list:

Your message dated Wed, 30 Sep 2009 23:21:34 -0400 (EDT) with subject
...  has  been  submitted to  the  moderators  of the THURSDAY-SUPPER list.

That response was posted to the list immediately, without moderation 
being applied to it.  I would have thought Listserv would never accept 
mail from the list address, either due to loop detection or some other 
rule.  Is there a way to prevent that from happening in the future? 
We're running Listserv 15.0, but I hope there's a solution short of 
upgrading.

Peter