After 3 months moderating comments manually, I had to introduce a captcha due to an exponential increase of the spam in the last month. After spending many hours trying to integrate recaptcha, I finally found a post on Code Capers which was exactly what I needed. Thank Michael.
It's simple, a little less secure than recaptcha but should be good enough.
Sorry for the legetimate users but you know like me it's a necessary evil.