BuddyPress.org

The safest way is to go through the UI on the front end and manually delete all the content items (forum posts, etc) and activity items, and then to delete all the test members from the Dashboard. I don’t suggest selective deletion directly from the database unless you really know what you’re doing.

Or you could go with a fresh install, by dropping all the tables in your installation, clearing your .htaccess and wp-config.php files, and going through the WP setup process again. You’ll also want to remove user-specific data like avatars, which on WPMU are in blogs.dir. As long as you don’t delete any plugin or theme files, you should be able to recreate your site pretty quickly – though your settings will be lost in the process, and will have to be recreated.