unallowable

As an adverb unallowably

is in a way, or to a degree, that cannot be allowed.

As an adjective unallowable is

not allowable.

As adjectives the difference between allowable and unallowable

is that allowable is appropriate; satisfactory; acceptable while unallowable is not allowable.

As a noun allowance

is the act of allowing, granting, conceding, or admitting; authorization; permission; sanction; tolerance.

As a verb allowance

is to put upon a fixed allowance (especially of provisions and drink); to supply in a fixed and limited quantity.

As an adjective unallowable is

not allowable.