The Collaborative International Dictionary of English v.0.48:

Disaffirmation \Dis*af`fir*ma"tion\, n.
   The act of disaffirming; negation; refutation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disaffirmation
    n 1: the act of asserting that something alleged is not true
         [syn: denial, disaffirmation]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "disaffirmation":
   abjuration, abjurement, abrogation, absolute contradiction,
   annulment, contradiction, contrary assertion, contravention,
   controversion, countering, crossing, denial, disallowance,
   disavowal, disclaimer, disclamation, disownment, disproof,
   forswearing, gainsaying, impugnment, nullification, recantation,
   refutation, renunciation, repudiation, retractation, retraction,
   revocation