Protect workbook from Ex-employees

How can I protect by workbook from ex-employees and the competition from using (I am aware of the weak security measures Excel offers, but something is better than nothing)? Each worksheet is password protected to slow down novice Excel users. However, ex-employees know the password. I was thinking if there was a way to have a changing password (which only a couple people would know) or have the worksheet look for a hidden file on our network, that one of these methods (or another) might work.