Here are some other ways to say "prohibido" in English, depending on the context:
* Not allowed
* Not permitted
* Restricted
* Off-limits
* No entry
* Keep out
* Out of bounds
For example:
* Prohibido fumar - No smoking
* Prohibido el acceso a menores de 18 años - Under 18s not allowed
* Prohibido tomar fotos - No photography
It's also important to note that the specific wording used in a sign or notice will depend on the context and the desired level of formality.