"This document describes how malicious or unwanted comments ('comment spam') affect weblogs, the techniques spammers use to abuse weblogs, and the tactics that can be used to prevent and defend against these attacks. Also included is a review of the strengths and weaknesses of each tactic, instructions for implementing them on your weblog and ones which we recommend for the best protection."
Well it should be called of course, the "Six Apart Guide to defeating comment spam" but at any rate, I found a tip there which, for now, seems to have dealt with the problem. If you use Movable Type you might be interested in the techniques.
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