BuddyPress.org

I was having this same issue, and this awesome guy linked to a plugin for me. Check out my original post with his link in it: https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/how-can-i-better-filter-spam-users-on-my-buddypress-site/

The nice thing about the plugin is that is also lets you ask users if they want to join your MailChimp email newsletter list when they sign up.

I am so looking forward to not having to deal with hundreds of spam users now!