WordNet (r) 3.0 (2006):

beg off
    v 1: ask for permission to be released from an engagement [syn:
         excuse, beg off]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "beg off":
   abandon, back out, be unmoved, be unwilling, cry off, decline,
   decline to accept, depart from, disagree, disallow, discard,
   disclaim, dissent, drop out, evacuate, forsake, go back on,
   hold out against, jettison, jilt, leave, leave behind, leave flat,
   maroon, negate, negative, not buy, not consent, not hear of,
   not think of, pull out, quit, quit cold, refuse, refuse consent,
   reject, renege, repudiate, resist entreaty, resist persuasion,
   say goodbye to, say nay, say no, stand aloof, stand down,
   take leave of, throw over, turn down, vacate, vote nay,
   vote negatively, withdraw