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]