Over the top
Security is good and bots annoy the hell out of me as well, but this is really the case of going over the top. I tried to register to the forums of the popular Mac torrent software Transmission. However, I got this captcha to make sure I was human:
I challenge you to read it. I could make out 2NVR and a T at the end... but it's supposedly six characters. I don't consider myself visually impaired - as the site suggests quite cheekily - or otherwise challenged, yet after three tries and three different but equally frustrating captchas I was told to try again later.