It´s a double choice between " You couldn´t / might not smoke on the bus", I know you can say you can´t smoke to prohibit something, or may not and couldn´t sounds terrible. I wonder if in BE, they say might not and what is the implication. Thanks.
Look at this :used for asking for or giving permission 2a. SPOKEN used for very politely asking permission to do something: Might I ask the president a question? I wonder if I might use your telephone. John asked if he might accompany me, and I agreed. 2b. FORMAL if someone said in the past that you might do something, they gave you permission to do it: Mr Binks had said that we might borrow his pickup truck.
To me, "you might not smoke on the bus" is not a prohibition, but a statement of probability, meaning the same as "maybe you won't smoke on the bus".
Also, "couldn't" sounds terrible because it's the wrong tense... in this case the only ones that sound right to me are "you may not" (formal), "you can't" (informal).