The Collaborative International Dictionary of English v.0.48:

Unreturnable \Unreturnable\
   See returnable.

Moby Thesaurus II by Grady Ward, 1.0:

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