If your Buzz course is connected with your Google Drive account, you can sign out of Google Drive from Buzz by opening the User Menu and clicking Logout of Google Drive.
How do we do this in the new UI?
This is currently not possible to do in the new UI. This link is a shortcut to a Google page (https://accounts.google.com/logout) that automatically sends a user to be logged out from Google. Until it is introduced into the new Buzz UI, you could create a link to it as a custom menu entry.