Comment by regenschutz

Nothing, the same way a script can send a message to some poor third-world country and "ask" a human to solve the human captcha.

Nothing, hence the qualifying "so that it's at least a little harder for humans to infiltrate" part of the sentence.

The captcha would have to be something really boring and repetitive like every click you have to translate a word from one of ten languages to english then make a bullet list of what it means.