In a game with polite forgiveness, if you do something that makes the game unwinnable, should the game immediately end (even if the action didn’t cause you to die or anything)? Or is it enough to simply state clearly that you’ve done something that makes the game unwinnable and let the player choose to restart?
Or how about idea C: I would probably just block the action. Depending on the action etc., your blocking message may not even need to explicitly address the issue of the game ending. Depending on where you’ve erected your fourth walls, this could be a great thing.
Really easy example: Dropping some crucial item off a cliff. The best solve to me is to stop that happening with a ‘you might /you’ll probably need that’ message.
From your question, my guess is you’re already working with a situation where it somehow won’t make sense to block the action. If it’s some life or death duel and you do the wrong thing, my style would be kill the person and offer undo. Edit - though you already suggested it doesn’t involve death.
In the original concept of a “polite” game, the game makes it completely clear that the player has made a mistake. “You have died” is of course one way to do that. I didn’t distinguish that case specifically, though.
In modern game convention, the very fact that the game continues may be taken as a promise that the game is still winnable. Or not. Depends on the audience and the game’s presentation.