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 [Codex.wordpress.org] Combating Comment Spam « WordPress Codex: Open and insecure proxy servers are sometimes used by spammers, since that will prevent them from being tracked down effectively. Genuine commenters will rarely, if ever, use an open or insecure proxy to browse your blog, so you can block comments originating from such proxy servers by doing the following:

 [Sixapart.com] Six Apart Guide to Combatting Comment Spam: Jay Allen's MT-Blacklist plugin for Movable Type provides content-filtering against a list of items that you can maintain (and a list that you can update automatically from a central list maintained at the Comment Spam Clearinghouse). MT-Blacklist offers numerous features in addition to its namesake blacklisting feature, such as blocking duplicated submissions, forcing moderation on older posts, limiting the number of URLs within the content of a single comment (spammers often attempt to send comments containing dozens or even hundreds of URLs;

Reflected tags on Technorati: Blog, Blogoptimization, Targeting Spam