[rattray]

Got it. Bummer.

I wonder if you could ask the user to trace a shape/pattern with their mouse? Or you draw a few animating dots with a canvas, and ask them to click the blue ones?

Fundamentally, though, you likely either piss people off by challenging their humanity, or violate their privacy by silently tracking their behavior, or break accessibility by evaluating the way they interact with your site against "normal" (bad for folks with screen readers, lynx, etc I'd assume).

[hombre_fatal]

Now come up with accessible versions of those tests.

[fragmede]

And then, harden it against bots that actually are humans being paid by the bot-writers, via Mechanical Turk.

[brokenmachine]

Recaptcha doesn't prevent that either, does it?