When I wrote a captcha script years ago, I included in the image the text: "Please only enter the text if you're on foobar.com", to deter people from relaying t

When I wrote a captcha script years ago, I included in the image the text: "Please only enter the text if you're on foobar.com", to deter people from relaying the image to a free porn site and getting horny teenagers to enter the captcha in return for porn.