Sure, but the point of a CAPTCHA is to pre-emptively detect the user as a human or computer. Waiting until the account sends a boat-load of spam, and then saying 'must've been a computer, let's disable the account' does nothing - the culprit has accomplished their task, and moved on. We want to be able to prevent the spam from being sent in the first place, somehow, and determining if it's a real human opening the account is (was?) a very good check.