BuddyPress.org

We installed the WP Super Captcha plugin on 3 different WP/BP high traffic sites and it cut the spam sign ups down by 80% immediately. Not a beautiful solution, by any means – but it does work somewhat. 80% is better than nothing.

Can be configured to show the captcha on the sign up page and login page – whichever you prefer, or both. Compliant for WordPress (with or without Multisite enabled) and BuddyPress configurations.

https://wordpress.org/extend/plugins/super-capcha/

Good luck!