You can not prevent anyone from running a report if they have access to the category and the report is added to the category. You set security on the category to give people access to a category, but not what they can see within that category.
Of course if you save a report in a personal folder and add that to the Mobile category, nobody but that user will be able to see that report since it is restricted to only that user (if your security is standard...)
Cheers,
Josh