It isn't amavisd's job to do that. Amavis is anti-virus. Spamassassin takes care of the spam. So, concentrate your efforts on configuring Spamassassin.
The way it should work: postfix (or fetchmail) gets the mail and routes it to amavis to check for virii/worms/trojans, then the mail goes to Spamassassin to check against whitelist/blacklist before returning the mail to postfix to distribute.
You can get some downloadable spamassassin blacklists from the spamassassin website.