The Collaborative International Dictionary of English v.0.48:

Banish \Ban"ish\ (b[a^]n"[i^]sh), v. t. [imp. & p. p. Banished
   (b[a^]n"[i^]sht); p. pr. & vb. n. Banishing.] [OF. banir,
   F. bannir, LL. bannire, fr. OHG. bannan to summon, fr. ban
   ban. See Ban an edict, and Finish, v. t.]
   1. To condemn to exile, or compel to leave one's country, by
      authority of the ruling power. "We banish you our
      territories." --Shak.
      [1913 Webster]

   2. To drive out, as from a home or familiar place; -- used
      with from and out of.
      [1913 Webster]

            How the ancient Celtic tongue came to be banished
            from the Low Countries in Scotland.   --Blair.
      [1913 Webster]

   3. To drive away; to compel to depart; to dispel. "Banish all
      offense." --Shak.
      [1913 Webster]

   Syn: To Banish, Exile, Expel.

   Usage: The idea of a coercive removal from a place is common
          to these terms. A man is banished when he is forced by
          the government of a country (be he a foreigner or a
          native) to leave its borders. A man is exiled when he
          is driven into banishment from his native country and
          home. Thus to exile is to banish, but to banish is not
          always to exile. To expel is to eject or banish
          summarily or authoritatively, and usually under
          circumstances of disgrace; as, to expel from a
          college; expelled from decent society.
          [1913 Webster]

WordNet (r) 3.0 (2006):

banish
    v 1: expel from a community or group [syn: banish, ban,
         ostracize, ostracise, shun, cast out, blackball]
    2: ban from a place of residence, as for punishment [syn:
       banish, ban]
    3: expel, as if by official decree; "he was banished from his
       own country" [syn: banish, relegate, bar]
    4: drive away; "banish bad thoughts"; "banish gloom"

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "banish":
   ban, blackball, blacklist, boycott, bump, can, cashier, cast out,
   cut, debar, deport, discharge, disfellowship, dismiss, displace,
   drive away, drive out, eject, evict, exclude, excommunicate, exile,
   expatriate, expel, extradite, fire, fugitate, lag, ostracize, oust,
   outlaw, proscribe, put out, reject, relegate, run out, rusticate,
   sack, send away, send down, send to Coventry, shut out, snub,
   spurn, thrust out, transport, turn out