Not sure of this is available on MAIL, but I suspect it is.
Using Entourage X - I just set the preferences to send any mail that is not from someone in my address book to the Junk Folder. I check the Junk Folder once a day and if there are any messages from people I want to inclue on the list, I just click and add them to the address book.
It sets up a "whitelist" rather than a "blacklist." The only junk items that make it through to my Inbox are an occasional peice of mail that's been sent by someone assuming the identitry of a person on my mailing list.