Soccer referee. Black shirt, black shorts, black socks. Whistle and red/yellow cards.
When somebody dumps a beer/drink, you blow the whistle and they are shown a yellow card for bad play. Write the costume in the book, and display the yellow card to the party. It's pretty funny. Red cards are used for peeing outside (illegal in the city), vomiting, or getting caught making out. Red wine on a white carpet is also a red card.