The Collaborative International Dictionary of English v.0.48:

Obstructionist \Ob*struc"tion*ist\, n.
   One who deliberately hinders progress; one who obstructs
   business, as in a legislative body. -- a. Of or pertaining to
   obstructionists; causing obstruction; as, obstructionist
   tactics. [Recent]
   [1913 Webster]

WordNet (r) 3.0 (2006):

obstructionist
    n 1: someone who systematically obstructs some action that
         others want to take [syn: obstructionist, obstructor,
         obstructer, resister, thwarter]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "obstructionist":
   bitter-ender, brawler, defendant, diehard, disputant, dissentient,
   dissident, filibuster, filibusterer, frustrater, hinderer, impeder,
   intransigent, irreconcilable, last-ditcher, litigant, marplot,
   naysayer, negativist, noncooperator, objector, obstructer,
   obstructive, opposer, oppositionist, plaintiff, protester,
   quarreler, resister, scrapper, thwarter, wrangler