The Collaborative International Dictionary of English v.0.48:

Preclude \Pre*clude"\, v. t. [imp. & p. p. Precluded; p. pr. &
   vb. n. Precluding.] [L. praecludere, praeclusum; prae
   before + claudere to shut. See Close, v.]
   1. To put a barrier before; hence, to shut out; to hinder; to
      stop; to impede.
      [1913 Webster]

            The valves preclude the blood from entering the
            veins.                                --E. Darwin.
      [1913 Webster]

   2. To shut out by anticipative action; to prevent or hinder
      by necessary consequence or implication; to deter action
      of, access to, employment of, etc.; to render ineffectual;
      to obviate by anticipation.
      [1913 Webster]

            This much will obviate and preclude the objections.
                                                  --Bentley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

preclude
    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: make impossible, especially beforehand [syn: preclude,
       rule out, close out]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "preclude":
   anticipate, avert, avoid, ban, bar, bar out, block, blockade,
   cease, check, count out, cut off, debar, deflect, deny, deter,
   disallow, discontinue, discourage, dishearten, embargo, enjoin,
   estop, exclude, exclude from, fend, fend off, forbid, foreclose,
   forestall, freeze out, frustrate, help, hinder, ignore, impede,
   inhibit, interdict, interfere with, keep from, keep off, keep out,
   leave out, lock out, obstruct, obviate, omit, ostracize, outlaw,
   pass over, prevent, prohibit, proscribe, quit, refuse, reject,
   relegate, repel, repress, repudiate, rule out, save, say no to,
   send to Coventry, shut out, stave off, stop, suppress, taboo,
   thwart, turn aside, ward, ward off