A: You can wait. If someone plays a card like a Rewind, you can choose to cancel the Power Action itself, and save that player the trouble of going any further, or you can wait until they choose a card, and then cancel the card they selected as they use it.

In the Back to the Future card game, similarly, you can stop someone from ever even going on a time trip by canceling the Time Machine card outright, or you can wait to see what they do with it, and then cancel that. Either way, the Time Machine goes in the trash and the Timeline remains the same.