Originally Posted by mrphillips
Like others have said, cold crashing doesn't take ALL the yeast out of solution. It's nearly IMPOSIBLE to take all the yeast out of solution. What you could do to stop fermentation is get yourself some potassium sorbate - it will stop the yeast from producing alcohol. It costs about a $1.50 at your homebrew store, and if you add it when your gravity is around 1.010-1.015, you shouldn't need to backsweeten your cider. Of course, this also means that you won't be able to carbonate because the yeast won't be able to eat up the priming sugar.
I would check the gravity to see where it's at, and if it's above 1.040, give it another week of fermenting, and you'll probably be close to your magic gravity number.
Sorbate doesn't work to stop an active fermentation, at least not in my experience. It only inhibits yeast reproduction, and in an active fermentation there are billions and billions of yeast that don't need to reproduce so it's ineffective in stopping an active fermentation.
It works well in a finished fermentation, when the cider/wine is totally clear and without any lees in the bottom, since there are less yeast in suspension. The sorbate then can be very effective to prevent renewed fermentation. It doesn't always work, as yeast are pretty tough buggers, but it's oftentimes effective if done right.