I have used potassium metabisulfite, the main component in campden tablets, a couple of times to stop fermentation in my cider. Although I now use Potassium Sorbate instead to reduce the amount of sulfites.
Campden tablets do not kill yeast but actually makes the environment inhospitable for the yeast. It does this by releasing sulfur dioxide into the must which prevents bacteria from growing and also acts as a strong antioxidant. When using it before bottling or to inhibit fermentation you need to add one tablet per gallon to get 150 ppm. If you are using potassium metabisulfite you will need 3/4 tsp to reach 150 ppm. You can also make a stock solution of 1/4 lb in 1 quart of water, then 1 tsp of the solution would get you 50 ppm, so 3 tsp would give you 150 ppm. When using campden tabs of potassium metabisulfite before fermentation to kill bacteria and arrest wild yeast you will need 75 ppm.
When I use potassium sorbate I first cold crash the yeast to promote flocculation and then add the sorbate. Potassium sorbate prevents the existing yeast from multiplying but doesn't stop them from fermenting. This is why I make sure to clarify as much as possible before kegging and bottling.