The Collaborative International Dictionary of English v.0.48:

Unalterable \Unalterable\
   See alterable.

WordNet (r) 3.0 (2006):

unalterable
    adj 1: not capable of being changed or altered; "unalterable
           resolve"; "an unalterable ground rule" [syn:
           unalterable, inalterable] [ant: alterable]
    2: of a sentence; that cannot be changed; "an unalterable death
       sentence"
    3: remaining the same for indefinitely long times [syn:
       changeless, unalterable]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "unalterable":
   adamant, adamantine, cast-iron, changeless, constant, dour, firm,
   fixed, flinty, grim, hard, hard-core, immovable, immutable,
   implacable, impliable, inalterable, incommutable, inconvertible,
   indefeasible, inductile, inelastic, inert, inexorable,
   inextensible, inextensile, inextensional, inflexible,
   insusceptible of change, intractable, intractile, intransigent,
   intransmutable, invariable, iron, irreconcilable, irresilient,
   irretrievable, irreversible, irrevocable, lasting, noble,
   nonelastic, nonreturnable, nonreversible, nonstretchable,
   permanent, relentless, reverseless, rigid, rigorous, rock-ribbed,
   steely, stern, stiff, stubborn, unaffected, unalterative,
   unaltered, unbending, unchangeable, unchanged, unchanging,
   uncompromising, undeflectable, undeviating, unextendible,
   unextensible, unflexible, ungiving, unlimber, unmalleable,
   unmodifiable, unmovable, unmoved, unpliable, unpliant, unrelenting,
   unremitting, unrestorable, unreturnable, unsusceptible,
   untractable, unvariable, unvarying, unyielding