What about old-good CAPTCHA? There are many ways, but all are annoying for normal users.
CAPTCHA only security is fantastic for spammers.
Or you can buy OCR software that plugs in to your bots; e.g. captchasniper.com
Captcha'd targets are usually higher-quality and more valuable as there is some economic cost of posting to it.
Thanks God for http://bugmenot.com!