How to close the workbook after a given amount of time?

For instance, one can get a trial activation key for 1 day, open the exe file and leave it open forever. It will continue to work!

Any Idea how to prevent it?

Solution: in VBA, you could set up a timer with Application.OnTime to close the workbook after a given amount of time. And forbid access to the VBA editor in XLS Padlock, so that customers cannot remove that code.