Mix sugar and 8 tablespoons of water in a pot. Melt the sugar at low heat while stirring. Then let it cool down. Add orange juice and Campari. Pour the liquid into a freezer-proof dish and cover and let it freeze in the freezer for at least 5 hours.
In between, whisk vigorously several times with a whisk or the hand mixer's hand blender. Moisten the edges of four dessert glasses with water and press into the white sugar. Fill the glasses with the sorbet just before serving. Decorate as you like with carambola and lemon balm