"Mustn't" is a word that is used to express prohibition or things that are not allowed. Alternatives to "mustn't" include "cannot," "shouldn't," "ought not to," "may not," "are forbidden to," "are prohibited from," "are banned from," "are not permitted to," "are not allowed to," "are precluded from," and "are restrained from." These synonyms vary in intensity and show the degree of prohibition that is applicable. If you want to express the prohibition in a more polite manner, you can use phrases like "it is not advisable," "it is discouraged," or "it is recommended to avoid." Regardless of which word you choose, they all convey the same message of prohibition and things that are not allowed.