Unfortunately, I can't use SpamAssassin as I'd like to reject spam before it's ever stored on my system. It's plugged in and filtering content, but as things currently stand, it can only be used to ignore email, not reject it. This means that spam is still stored on my system, and thus consuming my resources. This is a Bad Thing.
It turns out that this very thing is the subject of some flamage on the postfix-users mailing list. The next release should be out soon, but it will not have this functionality either. Maybe the next version will.