The Collaborative International Dictionary of English v.0.48:

Irreformable \Ir`re*form"a*ble\, a.
   Incapable of being reformed; incorrigible. --Joseph Cook.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "irreformable":
   beyond recall, beyond remedy, cureless, gone, graceless,
   immedicable, incorrigible, incurable, inoperable, irreclaimable,
   irrecoverable, irredeemable, irremediable, irreparable,
   irretrievable, irreversible, irrevocable, lost, past hope,
   past praying for, remediless, ruined, shriftless, terminal, undone,
   unmitigable, unredeemable, unregenerate, unrelievable, unsalvable,
   unsalvageable