Depending on the context:
* May: This implies permission or possibility. "You must wear a helmet" vs "You may wear a helmet if you want".
* Can: This emphasizes ability or choice. "You must go to work" vs "You can go to work, but you don't have to".
* Need not: This implies lack of obligation. "You must finish your homework" vs "You need not finish your homework tonight".
* Don't have to: This is a more informal way of saying "need not".
* Shouldn't: This implies something is not advisable. "You must be quiet" vs "You shouldn't be so loud".
Other options:
* Optional: This implies something is not mandatory.
* Unnecessary: This implies something is not required.
* Avoid: This implies something should be avoided.
The best antonym will depend on the specific situation and the intended meaning.