Formal:
* Unacceptable: This is the most direct opposite.
* Inadmissible: This implies something is not allowed or permitted.
* Reprehensible: This implies something is deserving of blame or condemnation.
* Objectionable: This implies something is offensive or disagreeable.
Informal:
* Uncool: This implies something is not trendy or desirable.
* Bad: This is a general term for something that is not good.
* No way: This is a colloquial expression for something that is completely unacceptable.
Specific:
* Improper: This implies something is not according to standards or rules.
* Inappropriate: This implies something is not suitable for the situation.
* Unsuitable: This implies something is not right or fitting.
The best opposite of "acceptable" will depend on the specific context and the nuance you want to convey.