Hi I've got a working fedora core 4 / PSA 8.2 / mail_scanner setup
that is working fine. Thanks to scott!
I host some email addresses that are forwarded externally. I'd like to delete spamassassin detected spam before forwarding it for these accounts. I don't do "local" delivery since my users don't have home directories. Their maildirs are located in /var/qmail/mailnames/domainname/user/. I've
got spamassassin setup to rewrite the subject line of detected spam.

What should I use to do this? procmail? maildrop? What process should I use?

maildrop if you're using psa-spamassassin or 4PSA. If you're using qmail-scanner, theres a delete setting (sa_delete) in /etc/qmail-scanner.ini