If only we had high resolution 3d printers or people who can carve lifelike portrait replicas out of stone! Oh, wait, we do. A bit more complicated, but if you think this is not way to simple to fool, you're out of luck.
If they can build a captcha that keeps out robots based on mouse movements, I'm pretty sure they can build a facial recognition system that can keep out prosthetics based on facial gestures.
Blood vessel scanner, or get out, see http://nsmartphone.com/fujitsu-lifebook-u745-review/