Re: Registration on bbPress boards
This plugin gives registrants a math test before they sign up:
https://bbpress.org/plugins/topic/human-test/
This lets you easily delete spammy signups (you should back up database first, of course!):
https://bbpress.org/plugins/topic/mass-delete-users/
This is a bit hardcore, but you can actually hold new registrations for approval:
https://bbpress.org/plugins/topic/approve-user-registration/
Hopefully one of those will do the trick!