(I) hope not
I would naturally expect that this would not happen or be true. A: "Don't worry, you won't have to pay for the additional service." B: "I hope not, seeing as I never agreed to it in the first place!"
(I) hope not.
a phrase expressing the desire and wish that something is not so. John: It looks like it's going to rain. Jane: Hope not. John: The Wilsons said they might come over this evening. Jane: I hope not. I've got things to do.