spoilsport

A spoilsport at Clarence House later said the royal couple had no firm plans to enter the race, adding: "It was said in jest."

No; I certainly don't want to be a spoilsport, and I'm glad I wasn't there—in my official capacity.

He must not be a spoilsport at feasts; his is no desert rle like John the Baptist's.

As elsewhere the use of the hyphen depends largely in the familiarity of the phrase; spoilsport, pickpocket.