In my experience the most effective anti spammer signup plugin has been SSS: https://wordpress.org/plugins/stop-signup-spam/ which works well with WPForo. Akismet and reCAPTCHA (V2) alone have proven to be vastly less effective.

If you want erect an iron curtain around your forum and you are a power user you should consider implementing something like ZB Block:  http://zb-block.net/zbf/ or CIDRAM:  https://github.com/CIDRAM/CIDRAM

Yes, I tried the built-in recapture.

It sent me in circles trying to log in myself, page after page image after image of buses signs and shopfronts.

In the end, it just kept rejecting me but was letting bogus registrations through.

🤨