BuddyPress.org

Best look in the WPMU forums for things (plugins) to help. Off the top of my head, some of the better ones are WP-Hashcash, Signup Question and Admin confirm new blogs. Hashcash is available at https://wordpress.org/extend/plugins and the others are at http://wpmudev.org/plugins/

Loads of good ideas though in the WPMU forums and that is the best place to start looking.

Trent