Is there anyway to block (or mark as spam) e-mails that have a matching line in the header?
In 2.1.4 you can use custom spam prefs to adjust the scores of an email:
http://spamassassin.apache.org/full/3.1.x/doc/Mail_SpamAssassin_Conf.html
Simplest thing to do would be to use the “blacklist_to” and “blacklist_from” options, found uner the “WHITELIST AND BLACKLIST OPTIONS” section (http://spamassassin.apache.org/full/3.1.x/doc/Mail_SpamAssassin_Conf.html)
There’s also the possibility for defining custom tests/rules too, if you need more control.
http://spamassassin.apache.org/full/3.1.x/doc/Mail_SpamAssassin_Conf.html#rule_definitions_and_privileged_settings