THESAURUS
nonreturnable
changeless, constant, immutable, incommutable, inconvertible, indefeasible, inert, insusceptible of change, intransmutable, invariable, irretrievable, irreversible, irrevocable, lasting, noble, nonreversible, permanent, reverseless, unalterable, unalterative, unaltered, unchangeable, unchanged, unchanging, undeflectable, undeviating, unmodifiable, unremitting, unrestorable, unreturnable, unsusceptible, unvariable, unvarying