In my experience comment spam on individual blog/cms (like a wordpress based site) are fairly easy to spot, typically they’ll be riddled with obvious keyword links to viagra, loans, porn etc. Even without some self learning spam detection system, simply blocking comments with more than a couple of links can filter out a lot of this stuff, and of course making all comments go in to a moderation queue ensuring they are manually checked by a human will stop all such comments.
Smarter Spammers
Smarter spam will not be so blatant and attempt to mimmick a genuine comment. This is actually quite easy to do – a generic post along the lines of “Thanks, that’s really useful” would probably sound genuine on the majority of informative posts and if the post doesn’t contain any links then why suspect such a comment? The give away is the commentor’s url, assuming your comment system allows the commentor to add their url (and in particular without rel=”nofollow” on the link) arguably the spammer’s goal has been accomplished – a cheap link back to their site. Even with specific keyword link text, a link on a page relevant to the spammers desired keywords will be a good catch.
How To Stop It
If you are maintaining a single isolated blog, then you may not have a means of picking up on comments like this. Visiting the websites of the commentors can certainly help weed out the obvious offenders, though arguably this is a for of referer spam – getting you to visit the site!
If you maintain a number of sites, then you can spot this sublte breed of comment spam, for example just today I received 2 identical comments to 2 completely unrelated blogs linking to wahoo.com. Furthermore the blogs weren’t english language sites either, so an english comment is a dead give away.
One action that one can take is to make sure you’re using a third party comment spam tool, such as akismet and mark the comments as spam.
I’ve chosen to go a step further and mention that wahoo.com has comment spammed several of the sites I maintain – surprisingly a seemingly legitimate enterprise, not some cheap viagra selling outfit! I wonder if this is the work of a dodgy SEO outfit…
Update: I just noticed google has a filter on its results to try and hinder comment/guestbook spam.
Just do a search for anything involving the term guestbook e.g. hotel guestbook, website guestbook etc. and click on any result after the 10th page.
For example here’s a search for pigeon guestbook:
http://www.google.com/search?q=pigeon+guestbook&hl=en&start=100&sa=N