Wordnet 3.0
ADJECTIVE (1)
1. 
 not capable of being stopped; 
- Example: "as unstoppable as the wind"
WordNet (r) 3.0 (2006):
unstoppable
    adj 1: not capable of being stopped; "as unstoppable as the
           wind" [ant: stoppable]
Moby Thesaurus II by Grady Ward, 1.0:
23 Moby Thesaurus words for "unstoppable":
   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, unpreventable, unyielding