If you have Spamassassin working, but your main mailbox is on a remote Imap folder, you can use Roger Binns' Isbg (Imap Spam Be Gone). The software can be found at http://www.rogerbinns.com/isbg/ , and an introduction to its use can be found at http://www.stearns.org/doc/spamassassin-setup.current.html#isbg