WordNet (r) 3.0 (2006):

negativist
    n 1: someone who refuses to do what is asked or does the
         opposite of what is asked
    2: someone who is resigned to defeat without offering positive
       suggestions [syn: defeatist, negativist]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "negativist":
   Cassandra, Eeyore, bitter-ender, brawler, calamity howler, cynic,
   defeatist, defendant, diehard, disputant, dissentient, dissident,
   filibuster, filibusterer, frustrater, hinderer, impeder,
   intransigent, irreconcilable, killjoy, last-ditcher, litigant,
   malist, marplot, naysayer, nihilist, noncooperator, objector,
   obstructer, obstructionist, obstructive, opposer, oppositionist,
   pessimist, plaintiff, prophet of doom, protester, quarreler,
   resister, retreatist, scrapper, seek-sorrow, thwarter, worrywart,
   wrangler