I would ask them how they intend to represent the number 9, and how they would represent the additive or multiplicative inverse of 8 or 101. Then, one of two things would happen: Either the student would decide that the notational idea they came up with is impractical, and I would have dealt with the question just as quickly as if I had told them “no, because The Rules say so”. Or, the student would come up with answers to those questions, and develop a new system of notation for numbers, and would in the process learn a great deal about how systems of notation of numbers work.
You’re not going to be able to gotcha me by asking “But what if a student starts to think for themself?”. That’s the whole point: I want students to think for themselves. That is the primary fundamental purpose of education.