On phpRiot.com there is a two part series that takes a look at a few ways to help manage spam with PHP - Anti-spam techniques in PHP, Part 1 and Part 2.

In Part 1 he looks at two perspectives on how to protect your site from spammers - protecting the posters and protecting your site. He suggests a few ways to accomplish this, including: obfusication, rewriting email addresses, dynamically making the email addresses images, and, finally, using a form for contact instead of just an email link.

In Part 2 he takes it a bit further, looking into a few more ways that you can protect your site an its visitors including: CAPTCHA images, manual approval/moderation of the comments on the site, and text filtering of all input.

Blocking spam on a site (weblog or otherwise) has proved to be one of the more difficult things about the whole weblog movement. It's gotten to the point where anyone can plop a chunk of code onto their web host and be up and running in anywhere from 10 to 30 minutes. The real problems come, however, when a spammer decides to take advantage of the site...thankfully, there have been some advances in this department, but I think a good, easy, fast solution is still out there somewhere...