If only the guardian and the people he/she guides there can find the Source, then isn't the best way for him/her to protect it to die and not name a new guardian?
On a somewhat related note, was the Source hidden from MIB for all these years? He was taken there by Jacob and "reborn" inside. You'd think he'd know how to get back and, being electromagnetic in his natural state, would be able to withstand the radiation in the chamber below. If he could find it and knew what the result of removing the cork would be, then I wonder if he didn't destroy the island because he didn't want to die, or if he simply couldn't do it by himself. If so, that might be an explaination for the skeletons in the chamber. Maybe they were the MIB's candidates: people with special electromagnitic gifts like Walt's, but just not quite like Desmond's so they didn't succeed.
What do you think about either or both?