User registration spam is prevalent in BuddyPress.
One way to dramatically decrease signup spam is to restrict the email address domains that users can sign up with.
WordPress multisite has a native option called “Limited Email Registrations”. But this option requires you to enter the full email domain. For example, if you only wanted to allow .edu email addresses to register, this is not possible.
This plugin restricts registrations to the email domains that you are not specified in the blacklist and works in WordPress single-site and multi-site.
Plugin is a fork of r-a-y’s BP Restrict Signup by Email Domain.
Tested on BuddyPress 2.5, but should work all the way down to BuddyPress 1.6.