Formal:
* Unable
* Incapable
* Unfit
* Inept
* Unqualified
* Prohibited
* Forbidden
* Impossible
* Impractical
Informal:
* Can't
* Ain't able to
* Not able to
* No way
* Out of the question
* No chance
* Don't have the power to
* Beyond my reach
Stronger/More emphatic:
* Absolutely cannot
* Under no circumstances
* By no means
* In no way
* It's out of the question
* It's impossible
More specific:
* Refuse (to do something)
* Decline (to do something)
* Prevent (from doing something)
* Disallow (from doing something)
The best synonym to use depends on the specific situation and the desired tone.