No captcha will ever be 100%. While it may appear that everything is getting through, this will certainly not be the case (feel free to switch it off, and you will see the comparison pretty quickly).
We certainly wish we could come up with a solution which would stop all spam on a permanent basis. Unfortunately, these days, many of these spam bots are sophisticated and just circumvent any measures put in place. And this is when they are bots at all, as often they are actually people posting. The reality is, the only way to stop guest spam posting completely, is to remove guests from posting completely.
I wish I had better news for you on that. I've found them to be annoying myself at times.