The Collaborative International Dictionary of English v.0.48:

Disallow \Dis`al*low"\, v. t. [imp. & p. p. Disallowed; p. pr.
   & vb. n. Disallowing.] [Pref. dis- + allow: cf. OF.
   desalouer, desloer, to blame, dissuade.]
   To refuse to allow; to deny the force or validity of; to
   disown and reject; as, the judge disallowed the executor's
   charge.
   [1913 Webster]

         To whom coming, as unto a living stone, disallowed
         indeed of men, but chosen of God.        --1 Pet. ii.
                                                  4.
   [1913 Webster]

         That the edicts of C[ae]sar we may at all times
         disallow, but the statutes of God for no reason we may
         reject.                                  --Milton.
   [1913 Webster]

   Note: This verb was sometimes followed by of; as, "What
         follows, if we disallow of this?" --Shak. See Allow.

   Syn: To disapprove; prohibit; censure; reject.
        [1913 Webster]

WordNet (r) 3.0 (2006):

disallow
    v 1: command against; "I forbid you to call me late at night";
         "Mother vetoed the trip to the chocolate store"; "Dad nixed
         our plans" [syn: forbid, prohibit, interdict,
         proscribe, veto, disallow, nix] [ant: allow,
         countenance, let, permit]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "disallow":
   abjure, assert the contrary, ban, bar, be unmoved, be unwilling,
   beg off, belie, blackball, categorically reject, contest,
   contradict, contravene, controvert, counter, cross, debar, decline,
   decline to accept, default, deny, disaffirm, disagree, disapprove,
   disapprove of, disavow, disclaim, disfavor, dishonor, disown,
   disprove, dispute, dissent, dissent from, embargo, enjoin, exclude,
   exclude from, forbid, forswear, frown at, frown down, frown upon,
   gainsay, grimace at, hold out against, impugn, inhibit, interdict,
   join issue upon, keep back, levant, look askance at,
   look black upon, negate, negative, not accept, not admit,
   not approve, not buy, not consent, not go for, not hear of,
   not hold with, not pay, not think of, nullify, object, object to,
   oppose, ostracize, outlaw, preclude, prevent, prohibit, proscribe,
   protest, recant, refuse, refuse consent, refuse to admit,
   refuse to pay, refute, reject, renounce, repress, repudiate,
   resist entreaty, resist persuasion, retract, revoke, rule out,
   say nay, say no, say no to, shut out, stand aloof, stop payment,
   suppress, taboo, take back, take exception to, take issue with,
   think ill of, think little of, thumb down, turn down,
   view with disfavor, vote nay, vote negatively, welsh, withhold