Change theme for all Sub-sites in a Multisite Setup

I currently have a multisite setup with about 900 active bloggers. The setup also uses Buddypress. I want to use a single theme for everything from buddypress to sub-sites and groups. I want to create a consistent look across our network.

The problem is that everyone is using their own template on the sub-sites.

Is there an easy way to change all the existing sites to use one theme?

Thanks in advance for any suggestions.