Put sugar and water in a small saucepan and boil. Rinse oranges in hot water and dry with a kitchen towel. Finely grate orange peel and avoid zesting the white skin. Stir orange zest into sugar syrup and cook for 20 minutes until bubbly. Pour orange syrup through a sieve and leave to cool in a shallow metal bowl over a cold water bath. Squeeze the juice from oranges. Add Campari and orange juice to syrup. Pour orange mixture into a bowl and place in the freezer for 45 minutes. Stir with a fork. Place in the freezer again and leave for 2 ½ hours, stirring occasionally.
To serve, scoop granita onto dessert plates and garnish with kumquats and lemon balm leaves.