1) Person or group of people who make others stop having fun for whatever reason, usually out of jealousy or spite, but sometimes because said fun is against the law.
2) Another name for one's wife, instructor, parent(s), the legitimate authorities.
Schlarp, I would but the 'fun police' will probably tell us off.
Fortress: "No, that would be fun. Get back to work, robot!"
Fingers: "Damn fun police!"