Hi, I solved it like this:

In the WP menu bar: settings > general and then disable "Anyone can register"  Sorry, the pic is in German Language.

I had that disabled but people were still registering. Not sure how or why. If you do prevent 'anyone' from registering, how do people signup to the forums?

In relation to my previous post about WPSpamShield, it actually blocked everyone registering when I was using it. I had to switch to using Better Wordpress reCAPTCHA. And for that to allow a reCAPTCHA box on the login screen I had to allow 'anyone' to register.

Perhaps the WPforo team could provide clearer instructions here on how all of this works, or a link to existing guidance with a focus on those of us not super wordpress familiar.

Ok, my English is really far from perfect, but I understand what you mean. Maybe you can understand me 🙂 It works in my forum like this, see pic:

Vasilij Ratej is right, wpForo has it's own registration option.

Most of WP's user registration is  disabled, so we'll have to explain all WP users to turn it one after each wpForo installation. Forum must have a user registration option. If you want to protect spam registration you should also make sure the anti-spam solution works on wpForo forum registration page, not only on the default WordPress registration form.