lock sheet format

NO but here is a idea you can tinker on.

Set up a macro that runs in the background and if user tries to open the edit sheet format dialog box, then close it immediately. Don't ask me if this is possible or not.

What are you afraid of them changing?  Along the lines of Deepak's comment - perhaps you could setup a macro that upon Save of a drawing it reloads the default sheet format first so that any changes they've made are discarded.