The Collaborative International Dictionary of English v.0.48:

Riddance \Rid"dance\, n.
   1. The act of ridding or freeing; deliverance; a cleaning up
      or out.
      [1913 Webster]

            Thou shalt not make clean riddance of the corners of
            thy field.                            --Lev. xxiii.
                                                  22.
      [1913 Webster]

   2. The state of being rid or free; freedom; escape. "Riddance
      from all adversity." --Hooker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

riddance
    n 1: the act of removing or getting rid of something [syn:
         elimination, riddance]
    2: the act of forcing out someone or something; "the ejection of
       troublemakers by the police"; "the child's expulsion from
       school" [syn: ejection, exclusion, expulsion,
       riddance]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "riddance":
   abandonment, abjuration, break, breakout, cession, clearance,
   deliverance, delivery, deportation, detachment, discard,
   discarding, disjunction, dispensation, disposal, disposition,
   dumping, ejection, elimination, emergence, eradication, escape,
   escapism, evasion, exile, expatriation, expulsion, extrication,
   flight, forgoing, forswearing, freeing, getaway, getting rid of,
   giving up, issuance, issue, jailbreak, jettison, junking, leak,
   leakage, letting go, liberation, liquidation, ostracism, outlawing,
   outlawry, outlet, prisonbreak, purge, recantation, release,
   relegation, relinquishment, removal, renunciation, rescue,
   resignation, retraction, sacrifice, scrapping, setting-free,
   severance, surrender, suspension, swearing off, vent, withdrawal,
   yielding