The Collaborative International Dictionary of English v.0.48:

Unusable \Unusable\
   See usable.

WordNet (r) 3.0 (2006):

unusable
    adj 1: not capable of being used [syn: unserviceable,
           unusable, unuseable]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "unusable":
   functionless, impractical, in disrepair, inapplicable, inoperable,
   inoperative, nonfunctional, nonutilitarian, otiose, out of order,
   out of whack, unconducive, unemployable, unfit, unhelpful,
   unserviceable, unsuitable, unworkable, useless