Actually “must (not)” has two meanings: 1. Speculation (the most common usage by far): If I say, “you must (not) be a native Russian speaker”, it’s clear from context that I’m merely speculating — the meaning is “I’m fairly certain you are (not) a native speaker.” Note that the contraction "mustn't" cannot be used for speculation. 2. Prohibition: “You must not talk in the library.” Requirement: “You must obey the speed limit.” Note that the contraction "mustn't" can be used for this meaning of prohibition. In the examples above, the meaning is fairly clear, but there are contexts where it might be ambiguous. “You must not work too hard” could easily have either meaning, although the most likely is speculation. As far as "You may do it but it is not necessary”, we do not use “must not” with that meaning in English. Instead, we say “you don’t have to.”

"You mustn't" means "it's prohibited" or it's not permitted. "You may do it but it is not necessary" = "You don't have to"

‘You must not’ is not ambiguous in English, in saying ‘you must not...’ to someone you are urging or commanding them not to do something, there is no room for uncertainty in what you mean, ambiguity might be introduced by uncertainty in the persons voice perhaps, but not the words. A more ambiguous phrase in English would be ‘you shouldn’t/should not do that’.

Выражение модальности в английском языке несколько более сложно чем в русском как Вы правильно отметили. Вопрос сложный и желательно его подизучить подробнее с какого нибуть учебника или пособия, тема "модальные глаголы"(modal verbs) дожна быть повсюду. Но суть в том что для подобный конструкций используют либо Continuous tense либо один из специальных модальных глаголов "can, could, may, might, will, would, shall, should, must" у коих нету прямых аналогов за использованием. В кратце отвечая на Ваш вопрос глагол 'must' находится почти у верхушки по строгости, этот глагол означает то что тот кто его говорит ожидает высокую ступень ответственности. 'Must not' можна перевести как запрещено или категорически запрещено.

It's interesting. You must not.. is not really common. The person knows that you can or might do it but knows that it would be bad: It means something like Please don't.. Or Its very important that you don't...