The Collaborative International Dictionary of English v.0.48:

Stultify \Stul"ti*fy\, v. t. [imp. & p. p. Stultified; p. pr.
   & vb. n. Stultifying.] [L. stultus foolish + -fy.]
   1. To make foolish; to make a fool of; as, to stultify one by
      imposition; to stultify one's self by silly reasoning or
      conduct. --Burke.
      [1913 Webster]

   2. To regard as a fool, or as foolish. [R.]
      [1913 Webster]

            The modern sciolist stultifies all understanding but
            his own, and that which he conceives like his own.
      --Hazlitt.
      [1913 Webster]

   3. (Law) To allege or prove to be of unsound mind, so that
      the performance of some act may be avoided.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stultify
    v 1: prove to be of unsound mind or demonstrate someone's
         incompetence; "nobody is legally allowed to stultify
         himself"
    2: cause to appear foolish; "He stultified himself by
       contradicting himself and being inconsistent"
    3: deprive of strength or efficiency; make useless or worthless;
       "This measure crippled our efforts"; "Their behavior
       stultified the boss's hard work" [syn: cripple, stultify]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "stultify":
   annul, asphyxiate, befool, bottle up, bring to nothing, buffer,
   cancel, cancel out, censor, check, choke off, clamp down on,
   come to nothing, constipate, cork, cork up, counterbalance,
   crack down on, crush, damp down, deaden, discourage, drown, dull,
   dupe, enfeeble, extinguish, frustrate, gag, gull, hold down,
   impair, infatuate, inhibit, invalidate, jump on, keep down,
   keep under, kill, muzzle, negate, negativate, negative, neutralize,
   nullify, offset, pour water on, put down, put on, quash, quell,
   quench, repress, restrain, shut down on, silence, sit down on,
   sit on, smash, smother, squash, squelch, stagnate, stanch, stifle,
   strangle, stunt, subdue, suffocate, suppress, throttle, thwart,
   trammel, undo, vitiate, void, weaken