...
But I really really want to do it anyway!
Don't say we didn't warn you
This page – "So you want to delete all your spam!" http://www.exit0.us/index.php/So%20you%20want%20to%20%27delete%27%20all%20your%20spam%21 tells how to configure procmail + SpamAssassin to delete email that looks like spam.
...
How can I configure procmail to bounce messages flagged as spam?
Don't bounce to the 'From-:' addresses. They're almost always forged . If you're own address has been abused by spammers as From-address you know what I'm talking aboutin spam messages, and will cause other people (those whose addresses have been used) a great deal of trouble.