Formal:
* Inapplicable: This word implies that something is not relevant or suitable for the situation.
* Unusable: This is a direct synonym for "cannot be used."
* Prohibited: This suggests that something is forbidden or not allowed.
* Invalid: This word implies that something is not legally or technically correct.
* Obsolete: This word suggests that something is outdated or no longer in use.
Informal:
* No good: This is a casual way of saying something is not usable.
* Out of the question: This phrase suggests that something is not even a possibility.
* Not an option: This is similar to "out of the question," but it may suggest that there are other options available.
* Off-limits: This phrase implies that something is restricted or unavailable.
* No way: This is a very informal way of saying that something is not possible.
More Specific:
* Incompatible: This word suggests that something doesn't work with something else.
* Ineffective: This word suggests that something doesn't produce the desired results.
* Defective: This word implies that something is faulty or broken.
* Restricted: This word implies that something is limited or controlled.
The best word to use will depend on the specific context and the desired level of formality.