Comment Spam
We've all seen that comment spam is becoming a serious problem. Particularly on Movable Type weblogs, where the generated pages are all very similar in structure and semantics, spammers are abusing comment systems to increase their rank on Google.

Even more frustrating than the spamming problem is the fact that there isn't a simple solution that will work for everyone and that all options have their own sets of pros and cons. During the past couple of months, we've been throwing around ideas at Six Apart about the best ways to combat spammers.

Readers of your weblog must register before posting to your weblog.
Before someone can post a comment to your weblog, they must register with your site.

For many webloggers, this solution is not ideal. Informal polling of webloggers has revealed that many do not want to require someone to register before posting. It usually discourages conversations from forming and is a barrier for open discussion. Additionally, without federation, logins on multiple weblogs become unmanageable.

While we do plan on integrating comment registration into Movable Type Pro (which we'll be talking about in more detail very soon), it's an option that serves a different purpose than just blocking spam. If you want to prevent links to explicit pornography from appearing on your site, you shouldn't have to be required to turn on comment registration.

Comments require approval before being posted
When a comment is posted, you can receive an email that provides a clickable link you must visit before the comment can be posted on your site.

For webloggers with a small amount of readers, this solution may be ideal. However, if you receive a good deal of comments, it's a solution that doesn't scale. Additionally, it may ruin the spontaneity of discussion.

Image comprehension technology
Before a comment can be posted on a weblog, human eyes must enter a code that, ideally, is not readable by a computer.

This solution is not feasible because of accessibility issues. Additionally, spammers seem to be searching with bots and entering spam manually.…

http://www.sixapart.com/log/2003/10/comment_spam.shtml