Formal:
* Prohibited: This is a very formal and straightforward synonym.
* Outlawed: This implies that the action or item is illegal.
* Forbidden: This emphasizes that something is not allowed.
* Interdicted: This suggests a formal, official order to stop something.
Informal:
* Off-limits: This is a more casual way to say something is not allowed.
* No-go: This is a slang term for something that is forbidden.
* Blacklisted: This implies that something is specifically excluded.
* Barred: This suggests that access to something is blocked.
* Restricted: This implies that access to something is limited.
Other possibilities:
* Censored: This specifically refers to the removal of material deemed offensive or inappropriate.
* Suppressed: This suggests that something is being hidden or kept from being known.
* Disallowed: This is a general term for something that is not permitted.
* Exiled: This can be used metaphorically to describe something that is excluded or rejected.
The best word to use will depend on the specific context and the nuance you want to convey.