General:
* Inappropriate
* Unsuitable
* Unfitting
* Unfavorable
* Objectionable
* Undesirable
* Reprehensible
* Unworthy
* Unpardonable
* Intolerable
Stronger:
* Outrageous
* Appalling
* Shocking
* Atrocious
* Abhorrent
* Deplorable
* Repulsive
* Vile
Formal:
* Inexcusable
* Unjustifiable
* Unwarranted
* Unconscionable
* Indefensible
Informal:
* No way
* Not on
* Out of the question
* Forget it
* Absolutely not
* Not a chance
More specific:
* For behavior: Disgraceful, disgraceful, disrespectful, rude, offensive
* For quality: Substandard, inadequate, deficient, faulty, shoddy
* For a request: Unreasonable, unrealistic, excessive, demanding
* For a situation: Unbearable, intolerable, untenable, unacceptable
The best synonym will depend on the specific context and the level of formality you need.