I can't seem get Spam Assassin to rewrite the subject lines with SPAM. This is on a Debian 7 box using Postfix, Dovecot, MySQL, and spamass-milter Here is the relevent configuration data from the config files:
/etc/spamassassin/local.cf
rewrite_header Subject *****SPAM***** (_SCORE_)
report_safe 0
required_score 5.0
use_bayes 1
bayes_auto_learn 1
/etc/postfix/main.cf
smtpd_milters = unix:/clamav/clamav-milter.ctl, unix:/spamass/spamass.sock
/etc/defaults/spamass-milter
OPTIONS="-u spamass-milter -i 127.0.0.1"
OPTIONS="-r -1"