Captchas are annoying, but I think they’re the only workable solution that anyone’s managed to come up with. However, there’s an ingenious implementation that does the job while simultaneously assisting various book scanning projects. The system provides two words to input, one being the actual captcha, and the other a word taken from a list of words that an OCR scanner failed to recognise. Promotes literacy, stops spam. And hey, this is a quasi-literary site, so it even fits the theme. Of course, you may consider the entire site to be spam, in which case no captcha system will help you.

Full explanations and details here, highly recommended.

reCaptcha