Potassium Metabisulphite. Sulphur based product used to kill wild yeast and bacteria in wines, ciders and even beer.
1 tsp per 5 gallons will net 50 ppm free SO2. Add to your must at least 24 hours to dissipate before pitching your yeast
or it will kill that, too. It can also be used to make a sanitizing solution (2oz per gallon for sanitizing)