this is theorhetical:
have your macro grab the necessary formulas and code stored as text from a hidden sheet for day to day use. designate the date and time you want them to be unusable. Have the macro always check the designated date against todays date, if the time has come, then have the macro delete the formulasand the hidden sheet, and force a file save. and close excel. unless they have a backup of the file this should do the trick.