The Collaborative International Dictionary of English v.0.48:

Resister \Re*sist"er\ (-?r), n.
   One who resists.
   [1913 Webster]

WordNet (r) 3.0 (2006):

resister
    n 1: someone who systematically obstructs some action that
         others want to take [syn: obstructionist, obstructor,
         obstructer, resister, thwarter]
    2: someone who offers opposition [syn: adversary,
       antagonist, opponent, opposer, resister] [ant:
       agonist]