NOMAIL is a distribution option  (meaning don't distribute anything to me
at all).  It has no effect  on incoming mail.  The only way to  prevent a
given system  from submitting is to  serve it off via  the SERVE command,
but this affects anything sent to the server (not just your list). On the
other hand,  if you want  to do this because  the site is  non compliant,
this is probably a good idea.
 
  Eric