The Collaborative International Dictionary of English v.0.48:

Annul \An*nul"\, v. t. [imp. & p. p. Annulled; p. pr. & vb. n.
   Annulling.] [F. annuler, LL. annullare, annulare, fr. L. ad
   to + nullus none, nullum, neut., nothing. See Null, a.]
   1. To reduce to nothing; to obliterate.
      [1913 Webster]

            Light, the prime work of God, to me's extinct.
            And all her various objects of delight
            Annulled.                             --Milton.
      [1913 Webster]

   2. To make void or of no effect; to nullify; to abolish; to
      do away with; -- used appropriately of laws, decrees,
      edicts, decisions of courts, or other established rules,
      permanent usages, and the like, which are made void by
      component authority.
      [1913 Webster]

            Do they mean to annul laws of inestimable value to
            our liberties?                        --Burke.
      [1913 Webster]

   Syn: To abolish; abrogate; repeal; cancel; reverse; rescind;
        revoke; nullify; destroy. See Abolish.
        [1913 Webster]

WordNet (r) 3.0 (2006):

annul
    v 1: declare invalid; "The contract was annulled"; "void a plea"
         [syn: invalidate, annul, quash, void, avoid,
         nullify] [ant: formalise, formalize, validate]
    2: cancel officially; "He revoked the ban on smoking"; "lift an
       embargo"; "vacate a death sentence" [syn: revoke, annul,
       lift, countermand, reverse, repeal, overturn,
       rescind, vacate]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "annul":
   abate, abolish, abrogate, abstract, annihilate, black out,
   blot out, bring to naught, bring to nothing, buffer, cancel,
   cancel out, come to nothing, counteract, counterbalance,
   countercheck, countermand, counterorder, delete, disannul,
   discharge, dispose of, dissolve, divorce, do away with, efface,
   eliminate, expunge, extinguish, frustrate, grant a divorce,
   grant an annulment, invalidate, make void, negate, negativate,
   negative, neutralize, nullify, obliterate, obtain a divorce,
   offset, outweigh, overbalance, override, overrule, part,
   put asunder, put away, quash, recall, recant, redress, remove,
   renege, repeal, rescind, retract, reverse, revoke, separate,
   set aside, split up, stultify, sue for divorce, suspend, thwart,
   undo, unmarry, untie the knot, vacate, vitiate, void, waive,
   wipe out, withdraw, write off