Sebastian Steinlechner contributed the following example based on information from:
  http://www.xray.mpe.mpg.de/mailing-lists/procmail/2001-01/msg00147.html


I thought it might be ok to just delete mail that was recognized as spam and 
then send the standard "No such user" message back to the sender. That way, 
real spammers will hopefully not put the address on their active list. In 
addition, mail that was tagged as spam but indeed is important mail will at 
least tell the sender that something went wrong so he's able to check back.

This helped me a lot:
 http://www.xray.mpe.mpg.de/mailing-lists/procmail/2001-01/msg00147.html
 
 So my .procmailrc now looks like this:
 :0fw
     | spamassassin
     :0e
     {
        EXITCODE=1
     }
     :0
     * ^X-Spam-Status: Yes
     {
       EXITCODE=67             # EX_NOUSER, see /usr/include/sysexits.h
       HOST=foo.bar.invalid.   # illegal hostname, should stop
                               # processing immediately
     }


Editor's Note:

As a small ISP, I can see this causing major issues.  My one comment would be
to consider using a check against the header, X-Spam-Level:.  Then you could 
return message that scored 8 or over and leave the others be.  Something like:

* ^X-Spam-Level: \*\*\*\*\*\*\*\*.*   #Tests for at LEAST 8 asterisks

instead of

* ^X-Spam-Status: Yes

I haven't tested this but the theory should hold up if not the exact example.

KAM