Well, you can make the display of the sheet dependent on the value of a variable and use an Input_box to govern that variable - that can be a password.
You have to put that Input_box on some other sheet.
That is nothing, however, that someone with a Named_CAL and access to the document_properties cannot revert - but there might be a possibility to make it more secure through the security_settings in the document_properties - if you make sure that everyone but yourself is automatically registered with USER privileges in that app.
P.S.: Yep, that works.
I don't remember exactly who told me, but there is a method where you have two apps and you as developer can open the one, type in a password and then open the other (in the same window) with ADMIN privileges. Anyone else can only open the second one and automatically has USER privileges and thus cannot edit sheet_properties or document_properties.