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]