I assume your using Excel 2007?
If yes, just follow the steps mrgroove posted here:Configure Password to Open or Password to Modify on Excel Spreadsheet
Although the demo shows word screenshots, the same is true for Excel. Basically what you want to do is set a MODIFY password.
This will allow users to open your excel spreadsheet with no password and make changes however they will need to save the changes to a NEW document unless they know the MODIFY password.
This is a good trick for creating a template. That's what I do at work.