Ban users by IP, IP Range, host name, user agent and referer url from visiting your WordPress's blog.
Anti-spam plugin – An advanced and free WP spam filter which protects your blog from 100 % of comment spam.
The plugin provides the next generation in captcha, BotProof Captcha 2.0, to a WordPress blog comment form. It is currently version 0.9.6.8 Beta.
Wordpress Security Plugin – Perform over 23 Basic Security Activities for your blog and get a free malware scan at the same time!
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SpiderSquash analyzes client behavior and determines whether or not a client is a legitimate user or a malicious bot.
Anti-Captcha is a transparent captcha solution which does not require any end-user interaction
3D objects-based CAPTCHA to get rid of spam in comments.
Simple question that blocks most spambots (and paid robots) by making them answer a common sense question