Bring water, Stevia, and star anise to a boil. Remove from heat and let stand for 5 minutes.
Discard star anise and cool the liquid. Stir liquid into juice. Freeze in a shallow metal pan, stirring occasionally (about every 40 minutes) with a fork, until liquid is frozen and granular, about 6 hours.
Just before serving, scrape with a fork, breaking up lumps.