The Collaborative International Dictionary of English v.0.48:

Kill-joy \Kill"-joy`\, n.
   One who causes gloom or grief; a dispiriting person; a
   spoilsport. --W. Black.
   [1913 Webster]

WordNet (r) 3.0 (2006):

killjoy
    n 1: someone who spoils the pleasure of others [syn:
         spoilsport, killjoy, wet blanket, party pooper]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "killjoy":
   Cassandra, Eeyore, calamity howler, crepehanger, cynic, damp,
   dampener, damper, defeatist, gloomy Gus, grouch, grump, malcontent,
   malist, negativist, nihilist, pessimist, prophet of doom,
   retreatist, seek-sorrow, sourpuss, spoilsport, wet blanket,
   worrywart