BuddyPress.org

You just need to find a capture that does work with the registration form. There’s plenty that do.

try si captcha

I would suggest you skip the captcha on registration and instead use something like
https://wordpress.org/plugins/buddypress-humanity/

or the plugin “good question”

There is another similar to those.. in my experience these work 100 times better in preventing spam registrations..

I also use “ip geo block”

These took my spam fighting down from hours a day to a minutes a week.