WordNet (r) 3.0 (2006):

unpreventable
    adj 1: not preventable; "unpreventable hysteria" [ant:
           preventable]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "unpreventable":
   certain, fated, fateful, indefeasible, ineluctable, inescapable,
   inevasible, inevitable, inexorable, inflexible, irresistible,
   irrevocable, necessary, relentless, resistless, sure,
   sure as death, sure as fate, unavoidable, uncontrollable,
   undeflectable, unstoppable, unyielding