All of the measures available to 'secure' the data in a spreadsheet can be broken by someone so determined.

My recommendation is to always keep a backup of your file in a secured location (removable drive, private folder on the server, etc.).

If the data integrity is that valuable or sensitive, placing the file in a "limited access folder" is good idea, as well. Your System Administrator should be able to set that up for you. Give Read Only rights to a few selected users.

Good Luck