The Collaborative International Dictionary of English v.0.48:

Insusceptible \In`sus*cep`ti*ble\, a. [Pref. in- not +
   susceptible: cf. F. insusceptible.]
   Not susceptible; not capable of being moved, affected, or
   impressed; that can not feel, receive, or admit; as, a limb
   insusceptible of pain; a heart insusceptible of pity; a mind
   insusceptible to flattery; new strains of bacteria
   insusceptible to penicillin. -- In`sus*cep`ti*bly adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "insusceptible":
   affectless, anesthetized, arctic, autistic, blunt, catatonic,
   chill, chilly, cold, cold as charity, cold-blooded, coldhearted,
   cool, dispassionate, drugged, dull, emotionally dead, emotionless,
   frigid, frosted, frosty, frozen, heartless, icy, immovable,
   impassible, impassive, inexcitable, insensitive, insentient,
   nonemotional, objective, obtuse, out of touch, passionless,
   self-absorbed, soulless, spiritless, unaffectionate, unemotional,
   unfeeling, unimpassioned, unimpressible, unimpressionable,
   unloving, unpassionate, unresponding, unresponsive, unsusceptible,
   unsympathetic, untouchable