WordNet (r) 3.0 (2006):

unstrap
    v 1: remove the strap or straps from [ant: strap]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "unstrap":
   cast off, detach, disengage, doff, free, liberate, loose, loosen,
   release, remove, take off, unbar, unbind, unbolt, unbridle,
   unbuckle, unbutton, uncage, unchain, unclasp, undo, unfasten,
   unfetter, ungag, unglue, unhandcuff, unhobble, unlace, unlash,
   unlatch, unleash, unlock, unloose, unloosen, unmanacle, unmuzzle,
   unpen, unpinion, unshackle, unstick, untether, untie, untruss,
   unyoke, weigh anchor