For strong negative certainty, only can't is the anwer, But I doubt if "must not" can be used, I don't think so. "must not" should be only for oblication like "You must not be home today = You shouldn't come home today", right?
While "must" can be used for both obligation and certainty. So why is the negative form of "must not" restricted to obligation only?
Q. That boy __________ be Jason. Jason has gone to Japan.
1)can 2)canít 3)must 4)must not 5)might
I don't think that "must" necessarily indicates "certainty", so much as an opinion on something that's highly likely: "This must be the bus for the city although it's running slightly early".
And "must not" is quite common in this sort of context: "Oops!Ö that mustn't have been the city bus, it didn't even stop".
In your example "can't" would be the obvious answer, but I wouldn't be surprised to hear "mustn't be Jason" in conversation.
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