I think as a future thing (not sure if this has been suggested before), there should be an option to disable/enable global sources when you open the Global Sources button, in a similar fashion with the check-boxes that the standard sources window has. When you disable a global source regardless if its enabled or disabled within the scenes, if its globally-disabled (but still saved so you dont have to remove it when you dont want it consuming resources) then the within-scene sources get disabled.