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We constantly manually review all referrals that come from sources other than the major SE's. It is time consuming, but I don't know of another way and I think it is worth it.
argv1900: You can ban referrals from any website via the .htaccess file, using the HTTP_REFERER field. I've only ever heard of people actually doing this in order to prevent the hotlinking of images, but in theory you can also prevent surfers from any website from accessing your regular pages.