The current CAPTCHA feature provided by Dolphin is severely basic and for the most part, useless against today's more advanced bots and other automated tools. With that said, I believe it would be both easier and more benneficial to replace the current CAPTCHA with reCAPTCHA. Perhaps both could even be made available, with the better one being the default option.
While this will not completely do away with the spam bots on Dolphin sites, it will most definately do a better job and keep the more basic ones away.