Have you tried implementing a question -> response registration type? (Not sure, been a while since I last registered here)
I had a similar problem on my page a while back and none of the CAPTCHAS worked. So I created a custom field on the registration page where I asked a question stating where the answer can be found. (Must be on a different page, on the top of the Help page should work) It's very easy for real humans to answer the question (open a new tap and voila) but bots will have a very problem going around that one. You can even easily generate a new Q&A every day making sure that the answer is nor being spread around or guessed easily.
On my page the amount of spam accounts went from daily to pretty much zilch. (Haven't had a bot in months)
Yes, there is a response question. However, the nature of SEO spamming is changing as well. Now, there are large rooms full of people who work for a few cents a day manually creating accounts and going thru the login procedure.
There are still lots of robots, but the manual process is more successful. They even post some text that may or may not be relevant, but its always obvious to readers.
We get maybe 3-7 spams thru a day, and these are handled pretty quickly. Thanks to those who report them. Those who pay SEO companies to do this are usually clueless, or they do get results and don't care. Even a big Fortune 500 company got caught hiring a shady SEO Spammer a couple of years back.