strictly speaking it doesn't kill them. from jack keller's website:
Sulfite, by itself, does not actually kill off the yeast. Instead it creates an environment increasingly hostile to yeast and deadly to most other microorganisms such as bacteria. Wine yeasts, however, can be quite tolerant of sulfites. For that reason a stabilizer is also added to the wine.
Potassium sorbate, sold as a chemical or behind a product name such as Sorbistat K, is a commercial wine stabilizer that should be used in conjunction with Campden. In other words, it works better with sulfites present than without, and it works better than sulfites alone. Potassium sorbate disrupts the reproductive cycle of yeast. Yeasts present are unable to reproduce and their population slowly diminishes through attrition.