The fool is us. I don't recall a true captcha mod for classic but it was discussed several times at ubbdev.com. Somebody did add a Captcha feature with a human question answer feature at some point in time but never followed up on the modification. If they did it was lost in time.
If upgrading is out of the question, then try to use what tools you have at hand.
Such as turn off features that allow bots from harvesting email accounts, close the board to guests, Email verification, Etc.
Other than that you can block by Ip not just by the classic control panel but by .htaccess if it is a repeat offender.
