But the OP was asking about how (as an author) they can stop their classes from being subclassed. I think this is totally legitimate from a design point of view
I think it's as legitimate as asking how I, as a car manufacturer, can stop the people who buy my cars from hanging fuzzy dice from the rear-view mirror. Or how I can stop them from keeping their muddy boots on the back seat.
It's none of my damn business. If I don't want them doing that, I shouldn't sell the car. If I really need the sale, but still don't want them doing it, I can document that it voids the warranty for their expensive leather seats.