unstoppable

from WordNet (r) 3.0 (2006)

unstoppable
    adj 1: not capable of being stopped; "as unstoppable as the
           wind" [ant: {stoppable}]

from 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