1) Pour grapefruit juice into a 23cm by 30cm nonreactive glass baking tray. Place in the freezer.
2) Freeze 6 to 7 hours, raking with a fork every hour to develop large, snow-like crystals and to stop it freezing solid.
To serve, use an ice cream scoop, and scoop into glasses. Pour 30ml of vodka over each ball. Garnish with mint and serve.