You can add a "password to modify" to a file. Do a file, save as and click the
Tools button, select "general options" and enter the "Password to modify"
there.
If you are using office2007 then
Click on Review Tab -> Select protect & Share workbook -> Check on sharing
with Track changes checkbox-> provide the password->click Ok
If you are using office2003 then
From menu select Tools -> Protection -> Check on sharing with Track changes
checkbox->Select protect & Share workbook->provide the password->click Ok