Simply adding sulfite and sorbate probably won't halt the fermentation, there just too many yeast cell at work. You need to reduce their population somehow first. Cold-crashing the entire batch and racking off the yeast cake before adding the stabilizers seems to be a pretty popular technique. The clearer you can get the cider, the fewer yeast cell will be in suspension.
Unless the manufacturer's direction say different, shoot for 1.25g/gallon sorbate and 50ppm Kmeta.