I would be curious to know how this failed, if they did try it. Are captcha automatable? You could make it a bit better by challenging every X actions, and have X decline with a failed captcha.
- captcha that isn't hard for humans is often easy to automate
- captcha that is hard to automate frustrates a lot of humans too
- and to add insult to injury you can "automate" the hard ones too by paying like $1 per 1000 captchas or something (think Amazon Turk like stuff)