I would start with a macro like this which will run automatically when the user saves the file.
Code:
Sub FileSave()
Dim aCC As ContentControl
For Each aCC In ActiveDocument.ContentControls
aCC.LockContents = Not aCC.ShowingPlaceholderText
Next aCC
ActiveDocument.Save
End Sub
I think it will need to be more complex though to handle check boxes. How is the macro supposed to know that a check box has been completed?
This protection method is only as strong as the user's knowledge. If they know how to unlock a CC then they can do so easily.