You can't possibly have a CAPTCHA in a system like Bitcoin. A CAPTCHA requires, among other things, that there be a computer generating it and keeping the answer to compare to. The question of "is this right?" is asked of whatever generated it and has the answer stored. A decentralized system has, well, no central point that can issue the CAPTCHAs and validate them -- if nodes are generating their own CAPTCHA, you'd be relying on the honor system, and there's nothing to stop modified software from bypassing the CAPTCHA check.