I have quite a problem with spam as well... but I've been able to control the problem recently with some IMAP Sieve rules. I simply sort all mail that doesn't have my address specifically in the to/cc/bcc fields into a Spam folder. I also did rules for emails containing "opt-in" "optin" and "ADV:". I have exceptions for mailing lists that I want to receive. It's cut almost 95% of my spam. I love going into my Spam folder and just deleting it all!!