The Capthca in this forum is pretty useless against actual bots, so I reocmmend the most simple solution: Use a plugin where you have to type an answer. Example: name one of the continents etc. Be sure that a human knows the answer and make sure, that the question is not a standard example. This solves the auto registration to 99% plus you can diable the stupid captchas .
No capchas present on this forum for at least 3 months... it's definitely a manual thing Still, somehow, they are getting through... thanks for the suggestion though.