Not sure if there is anything specific to SCM but in general this question has been asked many times on SCN over the years.
There is no standard solution that I'm aware of. As you've correctly noted, we can add a requirement routine, but it does not prevent manual output (there is a message displayed to that effect).
From what I recall from past discussions (use search), others either abandoned the idea or went with a modification (or enhancement). Don't recall anyone sharing more details though. Also since there is manual flag available you could print some special text on the form in such case.
But in general I do not recommend disabling manual output completely because sooner or later you will need it for legitimate reasons, e.g. re-printing in case of a printer failure.