Excel spreadsheets can be locked behind a password to prevent viewing or editing by non-authorised staff.
To set this up -
File > Save As > Browse > Tools > General options
Enter a password to 'Password to open' if you wish to prevent unauthorised viewing
Enter a password to 'Password to modify' if you wish to prevent unauthorised editing but to allow all to view.
Re-enter password in the box to ensure password is correct and not mistyped.
Save in desired location.