automatically close a workbook

I have an Excel application that is used primarily at one desktop in the
network, however, at certain times it's needed at a couple of other
workstations (for example, mine when I have some updated code to add in). The
users have been asked to close the file when they're done using the
application, but this doesn't happen on any kind of regular basis. Is there
any way to check for activity (similar to launching a screensaver) and close
the file after a certain period of INactivity?