Bee,
I recently read an article on the use of tablets and mobile phones in the wilderness. The take away is people have become emboldened to do marginal things because they have decided to carry this iffy technology to make up for a lack of skills; thus, making the life of SAR more "interesting".
Since the SPOT's cancel 911 does not give you feed back, I would have to say the user is responsible for personal contact with government official who can communicate the cancellation be it a volunteer patroling the trails with a radio or a sworn ranger. If this means ending a trip so be it.
