The Collaborative International Dictionary of English v.0.48:

Neutralize \Neu"tral*ize\, v. t. [imp. & p. p. Neutralized; p.
   pr. & vb. n. Neutralizing.] [Cf. F. neutraliser.]
   1. To render neutral; to reduce to a state of neutrality.
      [1913 Webster]

            So here I am neutralized again.       --Sir W.
                                                  Scott.
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   2. (Chem.) To render inert or imperceptible the peculiar
      affinities of, as a chemical substance; to destroy the
      effect of; as, to neutralize an acid with a base.
      [1913 Webster]

   3. To destroy the peculiar properties or opposite
      dispositions of; to reduce to a state of indifference or
      inefficiency; to counteract; to render ineffective; as, to
      neutralize parties in government; to neutralize efforts,
      opposition, etc.
      [1913 Webster]

            Counter citations that neutralize each other. --E.
                                                  Everett.
      [1913 Webster]

WordNet (r) 3.0 (2006):

neutralize
    v 1: make politically neutral and thus inoffensive; "The treaty
         neutralized the small republic"
    2: make ineffective by counterbalancing the effect of; "Her
       optimism neutralizes his gloom"; "This action will negate the
       effect of my efforts" [syn: neutralize, neutralise,
       nullify, negate]
    3: oppose and mitigate the effects of by contrary actions; "This
       will counteract the foolish actions of my colleagues" [syn:
       counteract, countervail, neutralize, counterbalance]
    4: get rid of (someone who may be a threat) by killing; "The
       mafia liquidated the informer"; "the double agent was
       neutralized" [syn: neutralize, neutralise, liquidate,
       waste, knock off, do in]
    5: make incapable of military action [syn: neutralize,
       neutralise]
    6: make chemically neutral; "She neutralized the solution" [syn:
       neutralize, neutralise]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "neutralize":
   abrogate, absorb the shock, annul, balance, break the fall,
   bring to nothing, buffer, cancel, cancel out, come to nothing,
   compensate, compensate for, conquer, counteract, counterbalance,
   countercheck, counterpoise, countervail, cushion, damp, dampen,
   deaden, defeat, delete, equalize, even, frustrate, invalidate,
   make up for, negate, negativate, negative, nullify, offset,
   overcome, override, overrule, redress, show consideration,
   show mercy, show pity, soften, soften the blow, square, stultify,
   subdue, suppress, thwart, undo, vitiate, void