To hide a sheet so that no body can unhide it without knowing the password for the Vb Project, simply open the VB Editor and change the visible property to 2 i.e. xlsheetveryhidden.
The sheet should disappear from the Unhide sheets command.
The only way to unhide it is to change the property back to 0 i.e. xlSheetNormal using the password to the VBProject.