Here's a small introduction on Postfix header (and body) checks:
http://www.postfix.org/header_checks.5.html
Basically you have to do something like this:
Code:
postconf -e 'header_checks = regexp:/etc/postfix/header_checks'
Now create the file
/etc/postfix/header_checks and put something like this into it:
Code:
/^X-Spam-Level: \*{15,}.*/ DISCARD
DISCARD will delete all messages with a spam score of 15 and above. You can choose another action than DISCARD, they are listed in the link I posted at the beginning.
Finally, restart Postfix:
Code:
/etc/init.d/postfix restart