Is it possible to disable a Magento extension from the backend without uninstalling it or disabling it via the code?
Two important distinctions here
A) Use System > Configuration > Advanced to shut off the module's output (module is still active)
B) To completely shut the module down and prevent execution of all its code, layouts, setups for both frontend and backend, go to app/etc/modules/*, open the module control file of the desired module. Set the line <active>true</active> to read false in order to actually shut the module off.
The first is fine for taking care of display, the second is needed when an unruly module has knocked your Magento installation offline and you need instant removal of its activity.
And of course the usual Magento Mantra 1) make change, 2) clear Magento cache...