Wet blanket

• (informal) someone who spoils the pleasure of others
• Defeat Johnson? Why? They say, he's a party pooper

Wikipedia Word definitions in Wikipedia
Killjoy , born Frank Pucci (on November 4, 1969), is the lead vocalist for the death metal band Necrophagia , and he is the only remaining original member of the group. He had his own solo band named Killjoy, which released one album, Compelled By Fear ...

Longman Dictionary of Contemporary English Word definitions in Longman Dictionary of Contemporary English
noun EXAMPLES FROM OTHER ENTRIES ▪ We wanted to do a sponsored dance after work but those killjoys in Head Office wouldn't let us.

Wiktionary Word definitions in Wiktionary
n. A person who is anti-fun, or prevents others from having fun