Obstructer

from 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}]

from The Collaborative International Dictionary of English v.0.48

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