The Collaborative International Dictionary of English v.0.48:

Prohibitive \Pro*hib"it*ive\, a. [Cf. F. prohibitif.]
   That prohibits; prohibitory; as, a tax whose effect is
   prohibitive.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prohibitive
    adj 1: tending to discourage (especially of prices); "the price
           was prohibitive" [syn: prohibitive, prohibitory]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "prohibitive":
   abusive, criminal, cutthroat, dear, deterrent, deterring,
   discouraging, ethnocentric, exacting, exceptional, excessive,
   excluding, exclusive, exclusory, exorbitant, extortionate,
   extravagant, fancy, forbidding, forestalling, gouging,
   grossly overpriced, high, immoderate, inadmissible, inflationary,
   inhibitive, inhibitory, inordinate, insular, insupportable,
   interdictive, interdictory, narrow, out of bounds, out of sight,
   outlandish, outrageous, overpriced, parochial, preclusive,
   preposterous, prescriptive, preventative, preventive, prohibiting,
   prohibitory, prophylactic, proscriptive, repressive, restraining,
   restrictive, seclusive, segregative, select, selective, separative,
   skyrocketing, snobbish, spiraling, suppressive, unconscionable,
   undue, unreasonable, unwarranted, usurious, xenophobic