• Add a CAPTCHA.
CAPTCHAs require users to read a bit of obfuscated text and type it back in to prove they're a human being and not an automated script. If your blog or forum system doesn't have CAPTCHAs built in you may be able to find a plugin like Recaptcha, a project which also helps digitize old books. CAPTCHAs are not foolproof but they make life a little more difficult for spammers. You can read more about the many different types of CAPTCHAS, but keep in mind that just adding a simple one can be fairly effective.

• Block suspicious behavior. Many forums allow you to set time limits between posts, and you can often find plugins to look for excessive traffic from individual IP addresses or proxies and other activity more common to bots than human beings.

• Block obviously inappropriate comments by adding words to a blacklist. Spammers obfuscate words in their comments so this isn't a very scalable solution, but it can keep blatant spam at bay.

• Use built-in features or plugins that delete or mark comments as spam for you. Spammers use automated methods to besmirch your site, so why not use an automated system to defend yourself? Comprehensive systems like Akismet, which has plugins for many blogs and forum systems and TypePad AntiSpam, which is open-source and compatible with Akismet, are easy to install and do most of the work for you.

• Try using Bayesian filtering options, if available. Training the system to recognize spam may require some effort on your part, but this technique has been used successfully to fight email spam.