The Collaborative International Dictionary of English v.0.48:

Unsustainable \Unsustainable\
   See sustainable.

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "unsustainable":
   baseless, controvertible, empty, groundless, idle, ill-founded,
   unattestable, unbased, unconfirmable, undemonstrable,
   undemonstratable, unfounded, ungrounded, unprovable,
   unsubstantiatable, unsupportable, unsupported, unsustained,
   untenable, unverifiable, unwarranted, vain, without basis,
   without foundation