WordNet (r) 3.0 (2006):

let off
    v 1: grant exemption or release to; "Please excuse me from this
         class" [syn: excuse, relieve, let off, exempt]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "let off":
   absolve, acquit, amnesty, backfire, blast, blow out, blow up,
   burst, bust, cast loose, charge, charter, chartered, clear, cock,
   decontaminate, demobilize, destigmatize, detonate, discharge,
   dismiss, dispense, dispense from, dispense with, drop, eject,
   except, excepted, exculpate, excuse, excused, exempt, exempt from,
   exempted, exonerate, explode, farm, farm out, favored, fell, fire,
   fire off, forgive, free, fulminate, give absolution,
   give dispensation from, go bail for, go off, grant amnesty to,
   grant bail to, grant immunity, grant remission, gun, gun for, hire,
   hire out, hit, immune, irresponsible, job, justify, lease,
   lease out, lease-back, lease-lend, lend-lease, let, let fly,
   let go, let go free, let loose, let out, licensed, load, nonpros,
   pardon, parole, pelt, pepper, permitted, pick off, pistol, plug,
   pot, potshoot, potshot, prime, privileged, purge, put on parole,
   quash the charge, release, released, relieve, remise, remit, rent,
   rent out, riddle, save the necessity, set free, set off, shoot,
   shoot at, shoot down, shrive, snipe, spare, spared, strike,
   sublease, sublet, take a potshot, torpedo, touch off,
   unaccountable, unanswerable, underlet, unhand, unliable, unsubject,
   vindicate, whitewash, withdraw the charge