As far as I know, there is no programmatic way to destroy an existing pizza. terraform destroy is implemented on the client side, by consuming the pizza.

How do they enforce idempotency? Does the driver come and take the pizza out of you if you change your order from 2 pizzas to 1?

I cannot emphasize enough how much you are actually going to be ordering a pizza. Please do not be surprised when you receive a pizza and a corresponding charge to your credit card.