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):

prohibit
    v 1: command against; "I forbid you to call me late at night";
         "Mother vetoed the trip to the chocolate store"; "Dad nixed
         our plans" [syn: forbid, prohibit, interdict,
         proscribe, veto, disallow, nix] [ant: allow,
         countenance, let, permit]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "prohibit":
   anticipate, arrest, avert, ban, bar, bar out, block, blockade,
   bridle, check, close out, constrain, contain, control, cool,
   cool off, count out, curb, curtail, cut off, debar, decelerate,
   deflect, deny, deter, disallow, discourage, disenable, dishearten,
   disqualify, dompt, embargo, enjoin, estop, exclude, exclude from,
   fend, fend off, foil, forbid, foreclose, forestall, freeze out,
   frustrate, govern, guard, hamper, help, hinder, hold, hold at bay,
   hold back, hold fast, hold in, hold in check, hold in leash,
   hold up, ignore, impede, inhibit, interdict, keep, keep back,
   keep from, keep in, keep in check, keep off, keep out,
   keep under control, lay under restraint, leave out, lock out,
   make impossible, obstruct, obviate, omit, ostracize, outlaw,
   pass over, preclude, prevent, proscribe, pull, pull in, refuse,
   rein, rein in, reject, relegate, repel, repress, repudiate,
   restrain, retard, retrench, rule out, save, say no to,
   send to Coventry, set back, shut out, slow down, snub, stave off,
   stop, straiten, suppress, taboo, thwart, turn aside, ward off,
   withhold