The Collaborative International Dictionary of English v.0.48:

Disbelieve \Dis`be*lieve"\, v. t. [imp. & p. p. Disbelieved;
   p. pr. & vb. n. Disbelieving.]
   Not to believe; to refuse belief or credence to; to hold not
   to be true or actual.
   [1913 Webster]

         Assertions for which there is abundant positive
         evidence are often disbelieved, on account of what is
         called their improbability or impossibility. --J. S.
                                                  Mill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disbelieve
    v 1: reject as false; refuse to accept [syn: disbelieve,
         discredit] [ant: believe]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "disbelieve":
   be skeptical, deny, discredit, distrust, doubt, eschew, gag on,
   misbelieve, mistrust, negate, not accept, not admit, not believe,
   not buy, not fall for, not go for, not swallow, question,
   refuse to admit, refuse to believe, refuse to credit, reject,
   resist believing, scoff, scorn, scout, suspect, unbelieve