Comment spam is something that every blog has to deal with. Black hat SEO have automated and semi-automated bots that can post thousands of spam comments across thousands of sites in a single day in an attempt to create SEO benefits for sites that are usually several steps removed from the spam comment.
Unfortunately spam comments do more than just pollute your blog posts; it can also hurt your site’s ability to rank, even if those comment links are nofollow links.
The SEOGadget Blog noted several years ago a pretty compelling correlation between spam comments and a loss of that page’s ability to rank — and once the spam comments were removed the rankings returned. This same behavior has since been identified on many other sites and the research is pretty clear.
Why Spam Comments Hurt Your Site
Who your site links to is a strong signal of what kind of site you are and what kind of “neighborhood” you’re in. In some ways this is a stronger signal of a spammy site than the links pointing to you. After all, anyone could pay for a bunch of crummy links to point to your site in an attempt to discredit you, but links from your site going out are things that you control.
Spam comments often combine two spam factors: they tend to include a lot of dangerous keywords that are often associated with spammy content: things like casinos, poker, gambling, viagara, prescription drugs, and various adult keywords. This is then combined with links to generally low-quality spammy sites that are totally associated with that spammy content.
The result is that, algorithmically, your site suddenly looks suspicious. It looks like it could be a site that was built solely to link to these other spammy sites; it looks like your site is connected to those spammy sites (and indeed with that comment spam it is). As a result Google will lower the rankings of your site, since it can’t trust it as much.
Monitor and Block Blog Spam
The only solution here is to do everything you can to stop spam comments from appearing on your blog, and more importantly to monitor the comments so you’re aware of when spam does eventually get through and can delete it. Remember that a lot of comment spam makes use of a combination of bots and people, so that actual people can fill out captcha forms to let the spam bot through.
For WordPress blogs you can see our post on plugins to stop comment spam — just remember that captchas alone are no longer enough.