I've made many wines, and I've never, ever had luck with stopping a fermentation once it's started. Campden definitely doesn't do it (some say it kills yeast, but it does not- wine yeast is not inhibited by sulfites, that is why we use campden in wine) and sorbate also won't kill yeast. Racking the cider and putting in cold storage will slow it down, but once it warms up it will restart.
What sulfites and sorbate do is to keep the yeast from reproducing. So, once the yeast is going, it's tough to stop. What I do is wait until it's done, add the sorbate and campden, and then wait three days and sweeten to taste. Fermentation won't restart, because the yeast can't reproduce. That's the only way I know of to have a sweeter cider.
Broken Leg Brewery
Giving beer a leg to stand on since 2006