Why would you care whether a user is a robot? Surely whether the actions of that user are desirable or not is determined solely by those actions and not whether the thing performing the actions is a human or a robot. It seems like a better idea to disallow bad actions than to disallow robots. There are also people farms who solve captchas (e.g. via porn sites who ask you to solve a captcha which they then input to another site, or by paying people $1 to solve X thousand capchas).