This is a possible solution to cut down on spam where users are required to interact with the site (guest books, forums etc.)
CAPTCHA: Telling Humans and Computers Apart Automatically
A CAPTCHA is a program that protects websites against bots by generating and grading tests that humans can pass but current computer programs cannot. For example, humans can read distorted text as the one shown below, but current computer programs can't:
The term CAPTCHA (for Completely Automated Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University. At the time, they developed the first CAPTCHA to be used by Yahoo.
A free, secure and accessible CAPTCHA implementation is available from the reCAPTCHA project. Easy to install plugins are available for WordPress, MediaWiki, PHP, Perl, Python, and many other environments. reCAPTCHA also comes with an audio test to ensure that blind users can freely navigate your site.
The exact version of Captcha you need depends on the software used to create your site, but that can be found by clicking on the main link above.
It is also important to realise that adding a Captcha question may make the website difficult or impossible for people with disabilities to use the website. More information on this can be found here.