Those who think captcha is fool proof are out of date, 10 years out.
I doubt a single person here thinks CAPTCHA is fool proof, and I certainly didn't read such a post. Also it is a system which may be around 10 years old, but has only been in common use for the past 5 year or so, and until someone comes up with something better it remains one option.
Anyway, I join others in thanking you for the work you do. I have worked on high volume forums in the past and it can be a nightmare.
Of course most of these bots - whether they be human bots, clever code, or a combination - nearly all want one thing - links. I would suggest you remove a member's ability to add a link to his/her signature, and manage how (or which) users can add links posts. Once you do that all the bots will (slowly) realise that it isn't worth their time trying to push through the system.