The Collaborative International Dictionary of English v.0.48:

Prohibit \Pro*hib"it\, v. t. [imp. & p. p. Prohibited; p. pr.
   & vb. n. Prohibiting.] [L. prohibitus, p. p. of prohibere
   to prohibit; pro before, forth + habere to have, hold. See
   Habit.]
   [1913 Webster]
   1. To forbid by authority; to interdict; as, God prohibited
      Adam from eating of the fruit of a certain tree; we
      prohibit a person from doing a thing, and also the doing
      of the thing; as, the law prohibits men from stealing, or
      it prohibits stealing.
      [1913 Webster]

   Note: Prohibit was formerly followed by to with the
         infinitive, but is now commonly followed by from with
         the verbal noun in -ing.
         [1913 Webster]

   2. To hinder; to debar; to prevent; to preclude.
      [1913 Webster]

            Gates of burning adamant,
            Barred over us, prohibit all egress.  --Milton.
      [1913 Webster]

   Syn: To forbid; interdict; debar; prevent; hinder.

   Usage: Prohibit, Forbid. To forbid is Anglo-Saxon, and is
          more familiar; to prohibit is Latin, and is more
          formal or official. A parent forbids his child to be
          out late at night; he prohibits his intercourse with
          the profane and vicious.
          [1913 Webster]

WordNet (r) 3.0 (2006):

prohibited
    adj 1: excluded from use or mention; "forbidden fruit"; "in our
           house dancing and playing cards were out"; "a taboo
           subject" [syn: forbidden, out(p), prohibited,
           proscribed, taboo, tabu, verboten]
    2: forbidden by law [syn: banned, prohibited]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "prohibited":
   absurd, banned, barred, closed-out, contraband, contrary to reason,
   debarred, deported, ejected, excluded, exiled, expelled, forbade,
   forbid, forbidden, hopeless, illegal, illicit, impossible,
   inconceivable, left out, liquidated, logically impossible,
   nonpermissible, not in it, not included, not permitted,
   not possible, off limits, out of bounds, outlawed, oxymoronic,
   paradoxical, precluded, preposterous, purged, ridiculous,
   ruled out, ruled-out, self-contradictory, shut out, taboo, tabooed,
   unallowed, unauthorized, under the ban, unimaginable, unlawful,
   unlicensed, unpermissible, unsanctioned, unthinkable, untouchable,
   verboten, vetoed