In the past few months, Mollom has done a very good job blocking spam on my site. Its accuracy has increased a lot since I first installed it, nearly no spam gets through even though there are hundreds attempts a day.

However, I've been seeing a different kind of spam that does get through. Comments that are left by humans, who can solve the captcha shown by Mollom. These comments are usually on-topic and at first glance appear to be legit, except that the homepage URL of the author links to a spam site. I enabled nofollow a long time ago, but that doesn't appear to make much of a difference anymore.

I've even seen comments that had no link in them at all. I'm not sure what the purpose of those messages is. Maybe they are just checking whether someone removes these comments, or maybe they are trying to confuse Bayesian filters. Either way they are pretty annoying.

I could enable comment moderation on this site, but that would make me the bottleneck as I don't check the queue that often. I noticed that nearly all of these comments show up on a limited number of pages. So I wrote a simple little module that allows me to enable comment moderation for individual nodes and enabled this on the pages that are targeted by these spammers.
This has been running for about a week now and has blocked all such messages. There have even been less attempts, possibly because of the warning message that appears in the comment form. Lets hope it stays this way so I can remove the nofollow tags on links.