Thank you, Martin and Duncan! Sorry I did not mention this information. I am using RH9 with Postfix. Maybe I can use Mailscanner.
Regardless of objections to using MailScanner with postfix (not supported by the postfix guys, but does seem to work for a lot of people) Don't *ever* use mailscanner's "bounce" feature.. It's broken beyond belief and makes your server into a DDoS tool. (Expands the scope of a Joe-job from a single-source DoS to a multi-source DDoS)
Many in the community, myself included, asked Julian to remove this feature, but at least one large site admin with a "I don't care who's site I DDoS, I want to bounce them because this makes mail 'reliable'..." attitude insisted he leave it in. So Julian deferred to adding a few extra hoops to enable the feature.
I myself have a standing policy of blacklisting with a 550 nastygram any server generating more than two such messages in a 24 hour period as a defensive measure.
If you want to reject mail in a less-damaging way, you need a MTA layer integration that gets called directly from the MTA's pipeline before the mail is accepted.
Since you're using postfix, your simplest way, as Duncan suggested, is amavis.
You can also make SA a content filter directly in postfix's master.cf. I'm not sure how this works, but I suspect this will allow direct rejection.
http://wiki.apache.org/spamassassin/IntegratedSpamdInPostfix
