Does your key have three positions? Mine does...lock, acc (where the car is not on, but the battery powers the accessories, and the steering is not locked) and on.
Since this has been in the news and my boys are learning to drive, I had them practice as if the car ran away on them. They turned the key to acc (which shut off the engine) and were able to coast and use the brakes to stop.
I have not given that a try and hope I never have to, however I understand the Prius doesn’t use a key so that wouldn’t apply in any case.