The Collaborative International Dictionary of English v.0.48:

isolationist \i`so*la"tion*ist\
   ([imac]`s[-o]*l[=a]"sh[u^]n*[i^]st), n.
   A proponent or supporter of isolationism; one who is
   isolationistic.
   [PJC] isolationist

The Collaborative International Dictionary of English v.0.48:

isolationist \i`so*la"tion*ist\, isolationistic
\i`so*la`tion*ist"ic\adj.
   1. of or pertaining to isolationism.

   Syn: isolationist.
        [WordNet 1.5]

   2. Favoring or supporting isolationism; -- of people.
      [PJC]

WordNet (r) 3.0 (2006):

isolationist
    adj 1: of or relating to isolationism [syn: isolationist,
           isolationistic]
    n 1: an advocate of isolationism in international affairs

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "isolationist":
   Diogenes, Hieronymian, Hieronymite, Timon of Athens, anchoress,
   anchorite, ascetic, bedridden invalid, cloistered monk,
   closet cynic, desert fathers, desert saints, eremite, free lance,
   free spirit, free trader, freethinker, hermit, hermitess, homebody,
   independent, individualist, invalid, latitudinarian, liberal,
   libertarian, libertine, loner, marabout, mugwump, neutral,
   nonpartisan, outcast, pariah, pillar saint, pillarist, recluse,
   rugged individualist, seclusionist, shut-in, solitaire, solitary,
   solitudinarian, stay-at-home, stylite, third force, third world