The Collaborative International Dictionary of English v.0.48:

Obstructer \Ob*struct"er\, n.
   One who obstructs or hinders.
   [1913 Webster]

WordNet (r) 3.0 (2006):

obstructer
    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]