You probably don't need to change the rules, Zimbra (spamassassin) already filters messages that contain those words. Unless you know what you're doing I wouldn't suggest writing your own rules as you might end up removing valid email. If you want to check the rules, have a look in this directory: /opt/zimbra/conf/spamassassin and extra rules are here:
SARE - SpamAssassin Rules Emporium - rulesemporium.com and some information on writing rules heer:
http://home.comcast.net/~mkettler/sa/SA-rules-howto.txt
If you are having a specific problem it might be an idea to discuss it before you start trying to write any new rules.