Probably y'all already know about this, but I am a slow learner.
I was getting lots of detailed comments being posted to my site that were well thought out, but seemed like possibly the person who was commenting had chosen the wrong post of ours upon which to comment - or maybe 3/4ths of the way through writing the comment, they had a brain cramp.
Their comments were about 90% related to the particular post, but there was something slightly off about them...
I finally decided to google the text of the comments and realized that some people are scraping youtube comments and using those as comment spam.
Then they started adding the comments to lots of blogs. I was surprised at how many "professional" blogs are showing these comments.
anyway, you all probably know this one. Just thought I would pass it along.
Most new Wordpress owners experience this. "Oh, I was so jazzed to see comments and then realized . . . " Don't feel dumb. It happens to all of us. They're probably not using the YouTube comments, it's just that their bots are probably pointed to both places. Moderate all your comments, that will help stop it, and enable Akismet (if you allow comments,) that will help.
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.