The Collaborative International Dictionary of English v.0.48:

Nullifier \Nul"li*fi`er\, n.
   One who nullifies or makes void; one who maintains the right
   to nullify a contract by one of the parties.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nullifier
    n 1: an advocate of nullification; someone who believes that a
         state can resist federal laws
    2: an official who can invalidate or nullify; "my bank check was
       voided and I wanted to know who the invalidator was" [syn:
       invalidator, voider, nullifier]