WordNet (r) 3.0 (2006):

negate
    v 1: be in contradiction with [syn: contradict, belie,
         negate]
    2: deny the truth of [syn: contradict, negate, contravene]
    3: prove negative; show to be false [syn: negate,
       contradict] [ant: affirm, confirm, corroborate,
       substantiate, support, sustain]
    4: 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]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "negate":
   abate, abnegate, abolish, abrogate, agree to disagree, annihilate,
   annul, be at cross-purposes, be contrary to, be unmoved,
   be unwilling, beg off, belie, blow sky-high, blow up, break,
   break off, bring to naught, bring to nothing, buffer,
   call into question, cancel, cancel out, challenge, clash, collide,
   come to nothing, conflict, contest, contradict, contravene,
   controvert, counter, counteract, counterbalance, countercheck,
   cross, decline, decline to accept, deflate, deny, differ,
   disaccord, disaffirm, disagree, disallow, disbelieve, disclaim,
   disconfirm, discredit, disprove, dissent, explode, expose,
   frustrate, gag on, gainsay, hold out against, impugn, invalidate,
   jangle, jar, jostle, misbelieve, mismatch, mismate, negativate,
   negative, neutralize, not admit, not believe, not buy, not consent,
   not hear of, not swallow, not think of, nullify, object, offset,
   oppugn, prove the contrary, puncture, quash, rebut, redress,
   refuse, refuse consent, refuse to admit, refuse to credit, reject,
   repeal, repudiate, resist entreaty, resist persuasion, revoke,
   say nay, say no, shake the head, show up, stand aloof, stultify,
   thwart, traverse, turn down, unbelieve, undercut, undo, vary,
   vitiate, void, vote nay, vote negatively