party pooper

noun1

1. •
   (informal) 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.

   • 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. — 1971