The Collaborative International Dictionary of English v.0.48:

Necessitarian \Ne*ces`si*ta"ri*an\, a.
   Of or pertaining to the doctrine of philosophical necessity
   in regard to the origin and existence of things, especially
   as applied to the actings or choices of the will; -- opposed
   to libertarian.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Necessitarian \Ne*ces`si*ta"ri*an\, n.
   One who holds to the doctrine of necessitarianism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

necessitarian
    n 1: someone who does not believe the doctrine of free will
         [ant: libertarian]