The Collaborative International Dictionary of English v.0.48:

Prevent \Pre*vent"\, v. t. [imp. & p. p. Prevented; p. pr. &
   vb. n. Preventing.] [L. praevenire, praeventum; prae before
   + venire to come. See Come.]
   1. To go before; to precede; hence, to go before as a guide;
      to direct. [Obs.]
      [1913 Webster]

            We which are alive and remain unto the coming of the
            Lord shall not prevent them which are asleep. --1
                                                  Thess. iv. 15.
      [1913 Webster]

            We pray thee that thy grace may always prevent and
            follow us.                            --Bk. of
                                                  Common Prayer.
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            Then had I come, preventing Sheba's queen. --Prior.
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   2. To be beforehand with; to anticipate. [Obs.]
      [1913 Webster]

            Their ready guilt preventing thy commands. --Pope.
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   3. To intercept; to hinder; to frustrate; to stop; to thwart.
      "This vile purpose to prevent." --Shak.
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            Perhaps forestalling night prevented them. --Milton.
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The Collaborative International Dictionary of English v.0.48:

Prevent \Pre*vent"\, v. i.
   To come before the usual time. [Obs.]
   [1913 Webster]

         Strawberries . . . will prevent and come early.
                                                  --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prevent
    v 1: keep from happening or arising; make impossible; "My sense
         of tact forbids an honest answer"; "Your role in the
         projects precludes your involvement in the competitive
         project" [syn: prevent, forestall, foreclose,
         preclude, forbid]
    2: stop (someone or something) from doing something or being in
       a certain state; "We must prevent the cancer from spreading";
       "His snoring kept me from falling asleep"; "Keep the child
       from eating the marbles" [syn: prevent, keep] [ant:
       allow, let, permit]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "prevent":
   abort, anticipate, arrest, avert, avoid, baffle, balk, ban, bar,
   block, check, control, curb, dam, debar, deflect, delay, deny,
   deter, disallow, discourage, dishearten, embargo, enjoin, estop,
   exclude, exclude from, fend, fend off, foil, forbid, foreclose,
   forestall, frustrate, hamper, help, hinder, impede, inhibit,
   intercept, interdict, interrupt, keep from, keep off, mitigate,
   obstruct, obviate, outlaw, preclude, prohibit, proscribe, refuse,
   reject, repel, repress, restrain, retard, rule out, save,
   say no to, shut out, slow, stave off, suppress, taboo, thwart,
   turn aside, ward, ward off