The Collaborative International Dictionary of English v.0.48:

Disannul \Dis`an*nul"\, v. t.
   To annul completely; to render void or of no effect.
   [1913 Webster]

         For the Lord of hosts hath purposed, and who shall
         disannul it?                             --Isaiah xiv.
                                                  27.
   [1913 Webster]

   Note: The prefix in this word an its derivatives is
         intensive, and not negative.
         [1913 Webster]