The Collaborative International Dictionary of English v.0.48:

Unsusceptible \Unsusceptible\
   See susceptible.

WordNet (r) 3.0 (2006):

unsusceptible
    adj 1: not susceptible to [syn: unsusceptible,
           insusceptible] [ant: susceptible]

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "unsusceptible":
   affectless, anesthetized, arctic, autistic, blunt, catatonic,
   changeless, chill, chilly, closed to, cold, cold as charity,
   cold-blooded, coldhearted, constant, cool, dispassionate, drugged,
   dull, emotionally dead, emotionless, frigid, frosted, frosty,
   frozen, heartless, icy, immovable, immutable, impassible,
   impassive, impersuadable, impersuasible, impervious, incommutable,
   inconvertible, indefeasible, inert, inexcitable, inflexible,
   insensitive, insentient, insusceptible, insusceptible of change,
   intransmutable, invariable, irretrievable, irreversible,
   irrevocable, lasting, noble, nonemotional, nonreturnable,
   nonreversible, objective, obstinate, obtuse, out of touch,
   passionless, permanent, reverseless, self-absorbed, soulless,
   spiritless, unaffectionate, unalterable, unalterative, unaltered,
   unamenable, unchangeable, unchanged, unchanging, undeflectable,
   undeviating, unemotional, unfeeling, unimpassioned, unimpressible,
   unimpressionable, uninfluenceable, unloving, unmodifiable,
   unmovable, unpassionate, unpersuadable, unpliable, unreceptive,
   unremitting, unresponding, unresponsive, unrestorable,
   unreturnable, unsuggestible, unswayable, unsympathetic,
   untouchable, unvariable, unvarying, unyielding