The Collaborative International Dictionary of English v.0.48:

Ostracize \Os"tra*cize\, v. t. [imp. & p. p. Ostracized; p.
   pr. & vb. n. Ostracizing.] [Gr. 'ostraki`zein, fr.
   'o`strakon a tile, a tablet used in voting, a shell; cf.
   'o`streon oyster, 'oste`on bone. Cf. Osseous, Oyster.]
   1. (Gr. Antiq.) To exile by ostracism; to banish by a popular
      vote, as at Athens. --Grote.
      [1913 Webster]

   2. To banish from society, by a general consent; to exclude
      from social, political, or private favor; to exclude from
      conversation or friendship; to shun; as, he was ostracized
      by his former friends. A person may be ostracized by a
      formal vote or by a widespread but informal agreement.
      --Marvell.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

ostracize
    v 1: expel from a community or group [syn: banish, ban,
         ostracize, ostracise, shun, cast out, blackball]
    2: avoid speaking to or dealing with; "Ever since I spoke up, my
       colleagues ostracize me" [syn: ostracize, ostracise]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "ostracize":
   avoid, ban, banish, bar, bar out, blackball, blacklist, blockade,
   boycott, cast out, categorically reject, cold-shoulder, count out,
   cut, cut off, debar, deport, disallow, disapprove, disapprove of,
   disfavor, disfellowship, displace, dissent from, embargo, exclude,
   excommunicate, exile, expatriate, expel, extradite, freeze out,
   frown at, frown down, frown upon, fugitate, grimace at, ignore,
   isolate, keep out, leave out, lock out, look askance at,
   look black upon, not approve, not go for, not hear of,
   not hold with, object, object to, omit, oppose, oust, outlaw,
   pass over, preclude, prohibit, proscribe, protest, reject,
   relegate, repudiate, rusticate, say no to, segregate, send away,
   send down, send to Coventry, shun, shut out, snob, snub, spurn,
   taboo, take exception to, think ill of, think little of, throw out,
   thrust out, thumb down, transport, view with disfavor