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]