Killjoy

Killjoy was a word generally applied to people who ruined other people's fun or happiness.

On Terra Alpha, the word was a cultural stigma, applied to people who rebelled against Helen A and her Happiness Patrol, being sad and wandering the streets alone rather than being happy and with others. One killjoy was lured by Silas P and was executed by the Patrol. (TV: The Happiness Patrol)

Tanith referred to the Seventh Doctor as a "po-faced killjoy." (PROSE: Falls the Shadow)