The Collaborative International Dictionary of English v.0.48:

Unhelpful \Unhelpful\
   See helpful.

WordNet (r) 3.0 (2006):

unhelpful
    adj 1: providing no assistance [ant: helpful]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "unhelpful":
   disregardful, forgetful, functionless, heedless, in disrepair,
   inapplicable, inconsiderate, inoperable, inoperative, mindless,
   nonfunctional, nonutilitarian, otiose, out of order, out of whack,
   respectless, thoughtless, unaccommodating, uncomplaisant,
   unconducive, uncooperative, unemployable, unfit, unheedful,
   unmindful, unobliging, unsuitable, unthinking, unthoughtful,
   unusable, unworkable