Protect a workbook from leaving the company

I need to protect a proprietary spreadsheet.  We've created what we suspect is a unique tool that helps us in our line of work (civil engineering).  We feel like it is something that gives us a competitive advantage, and we want to prevent it from being uploaded, shared, or spirited off by a departing employee (for instance).  

We want the spreadsheet to be readily available writable, copyable, etc for our staff, yet somehow protect it. I'm an IT guy and not a heavy excel user, and I'm hoping there are tools or best practices for this situation.  Any help would be greatly appreciated.