WordNet (r) 3.0 (2006):

obstructor
    n 1: someone who systematically obstructs some action that
         others want to take [syn: obstructionist, obstructor,
         obstructer, resister, thwarter]
    2: any structure that makes progress difficult [syn:
       obstruction, obstructor, obstructer, impediment,
       impedimenta]