The following steps will cause all Spam (as detected by Spam Assassin) to be deleted.  Note that SA has some false positives so this may delete some real non-spam mail.  This should only be used for accounts where the owner seldom reads ther mail.
  1. Create a ,procmailrc file containing:
    :0
    * ^X-Spam-Flag: YES
    {
      :0
      "| cat - >/dev/null"
    }
  2. Create a .forward file containing
      "|IFS=' ' && exec /usr/local/bin/procmail -f- || exit 75 # username"
    The last word in the line is the user's userid.
  3. Make sure all these files are owned by the user and readable, but not writable, by group and world.

To forward a copy of the non-SPAM email and also keep a copy in the current user's inbox, add the following lines at the end of the .procmailrc described above. 

:0 c
! someemail@some.other.place.com