The "are you human" is just a comparison between what response do you want and what the user says.
Those basic anti-spam methods are easily to hack. I will not really recommend use that at least if you have something like "check your inbox and click in the activation URL" like in the Chevereto create account page (from the main site, not the forums).
The tool that will really stop bots in things like contact forms is reCapcha which is free and not so hard to implement.