party pooper

(idiomatic) One who dampens fun; especially, one who declines to participate in or enjoy a party or some other group activity; one who does not agree with the opinions of a group.

Example: 1971, Robert Yoakum, "Scrooging up courage for a party," Eugene Register-Guardian, 19 Dec., p. 11A:
  Call me anti-social, call me a holiday-hater, call me Scrooge, call me a party-pooper—call me anything, but please don't call me about a party you're giving because I won't come.
2001, Tony Karon, "Who's a Racist? Can a UN Conference Decide?," Time, 1 Aug.:
  Still, it's not something that any country can easily walk away from—not even a Bush administration that has made a habit of being the party pooper in international forums.