While Captchas make our comments sections reasonably proofed against automated spam, lately we've had a spate of SEO wankers visiting us. These folks make carefully handcrafted spam, manually posted, that serve only to provide backlinks to their clients' webpages. The idea is that, by sprinkling this URL across many different blogs, they can trick Google et al into thinking that people all over the internet are casually linking to their website. They probably charge at least as much for this per hour as I do in my own consulting job.
I've just modified the text that appears over posts' comment forms to state that folks posting only for SEO reasons are wasting their time, since Zarf or I will delete the URLs from obvious SEO comments as soon as we see them. (We will also delete the comments entirely, if the commenter didn't even try to be on-topic with the related post.)