[fetchmail]filtering mail

Hello,
I currently use Fetchmail 5.8.10, Postfix, and Courier-Imap.  I am wanting to 
be able to bounce spam out of a list that I have.

Right now, if mail is sent directly to my domains, and is handled by postfix, 
it sends a bounce using a hashed access table, much like sendmail.  This table, 
however, does not effect what fetchmail brings in when it pops into another 
account.

What can I use as a mail filter?

Thanks!

jeff