'Designing out' spam
As the old idiom goes, prevention is always better than a cure.
This holds true more than ever with Spam, and more than ever prevention is definitely possible.
Mitigating the accessibility cost of CAPTCHA
CAPTCHA is the most effective anti-spam technique in use today, because it acts as a preventative. However this effectiveness comes at a heady price – seriously diminishing the accessibility of your website.
Tackling spam in Textpattern
Textpattern, a CMS that this very site uses, has fairly solid anti-spam built-in, but it can be improved with a little configuration.
Math CAPTCHA
My Mathematics based CAPTCHA. Highly accessible, difficult for spambots due to human language and random sums.
Regarding 'Inaccessibility of CAPTCHA'
Matt May, whilst at the W3C, wrote an excellent paper on the inaccessability of the most common forms of CAPTCHAs.
However, for many this type of spam-deterrent is the only effective method they can easily implement.