Or you can use this
   Send= Editor,Hold,Confirm
Be sure to omit the non-member option.  From the help:  "When the
"Confirm" option is enabled, the sender is required to confirm the
posting. When LISTSERV receives mail for the list, it sends an e-mail to
the sender requesting a confirmation. The confirmation can be
accomplished by replying to the confirmation with another e-mail
containing the text "OK" (the subject of the confirmation e-mail
contains a validation "cookie") or by clicking on the confirmation URL
provided in the e-mail."

Either way adds some small inconvenience.  Or you might explore the
unwanted messages in some detail to see if they share any trait you can
add to a Filter option maybe.

>>> [log in to unmask] 11/10/2005 3:38:56 PM >>>
One of my lists is Editor-Hold and approved messages are distributed to
our staff. Recently, we have had a few emails that must have had a
Return Receipt request on it because the following is a typical message
was sent automatically to the list, list owner, and original sender. The
staff are set to bypass moderation so approval was circumvented. Is
there a way to strip out return receipts for email processed by
Listserv? I suppose the easiest would be to remove the moderation
bypass?

Your message
   To:      [log in to unmask] 
   Cc:
   Subject: Fact Sheets
   Sent:    Sat, 8 Oct 2005 03:54:45 -0800
was deleted without being read on Thu, 10 Nov 2005 12:11:22 -0800


------------
Regards,
Randy Klumph
[log in to unmask] 

Teaching Research Institute
Western Oregon University
345 N Monmouth Ave
Monmouth, OR 97361

http://www.tr.wou.edu