The Collaborative International Dictionary of English v.0.48:

Unpersuadable \Unpersuadable\
   See persuadable.

WordNet (r) 3.0 (2006):

unpersuadable
    adj 1: not susceptible to persuasion [syn: unpersuadable,
           unsuasible]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "unpersuadable":
   blind, cautious, closed to, deaf, disinclined to believe,
   disposed to doubt, dogmatic, guarded, hard of belief,
   impersuadable, impersuasible, impervious, impervious to persuasion,
   inconvincible, incredulous, inflexible, leery, obstinate, positive,
   shy of belief, skeptical, suspecting, suspicious, unamenable,
   unconvincible, uncredulous, unimpressionable, uninfluenceable,
   unmovable, unpersuasible, unpliable, unreceptive, unresponsive,
   unsuggestible, unsusceptible, unswayable, unwilling to accept,
   unyielding, wary