Sorry for all this, it appears to be human spam as they are circumventing the captcha no problem. They have been circumventing our other spam protection as well effectively.
I've spent the morning cleaning up all the remaining spam, and improving our spam methods on the forum which should now be even more effective. I can't go into what we do exactly I'm afraid for obvious reasons, but it should be robust going forwards and easy for us to adjust to new attacks.
Working on solving this issue now.