The original node made me think of "I need a class that can't be used on Tuesdays. I could just check localtime in new(), but somebody could intentionally set their computer's clock wrong..."

I suppose there could be a class that the author knows would break if subclassed.

If that were the case, then I would put the checking at/around the point that I thought would break if it was subclassed. I wouldn't go the indirect route and try to prevent all forms of subclassing. And this would also avoid the obvious and wise questioning of "why do you want to make subclassing fatal?". I would also question a conclusion so broad as "this will break if subclassed". I expect such would more correctly be stated as "this will likely break if subclassed in the obvious way".

But that is based on a lot of speculation. It would be nice to know why such a bizarre (IMHO) requirement was formulated.