BuddyPress.org

1. Activate BuddyPress Home theme for main blog.

2. Site Admin -> Themes – Deactivate all themes including BuddyPress Home theme.

3. Site Admin -> Themes – Make sure News theme is enabled.

4. Find and activate the “New Blog Defaults” plug in. (Help ensure new blogs get the default theme you want.)

5. Configure “New Blog Defaults” and set default theme to your News one.

6. Create a new blog yourself, and you’ll be forced to use the only active theme, and cannot change to any other.