Kendo - 5:17 pm on May 20, 2012 (gmt 0)
Wordpress and others CMS systems are targeted because they are so out of the box and predictable. A few weeks ago I stumbled upon a couple of apps for SEO keyword spamming, and I think that these are your problem.
What they can do is crawl the web looking for Wordpress and other commonly used web solutions to find sites and forums related to your keyword. Then they will submit to those sites. Some of these apps can run automatically for days, and yes it works.
The appropriateness of the posted message can vary and it can depend on how clever the operator is when creating what is known as "spun" messages. Spinning is a technique that makes very message different from the next to evade detection by search engines.
Forget about Captcha as these softwares can read Captcha images that humans cannot. To prevent the onslaught a simple script that checks the referrer will work, especially if the referrer is not from your site.