Originally Posted by TipsyDragon
Potassium Sorbate doesn't kill yeast it just stops them from starting fermentation again. 1 campden tablet per gallon will kill the yeast dead but then there will be no yeast to condition the wine. if your worried about the wine being to dry i would suggest letting the yeast do their thing till they are done then add the Sorbate and back sweeten to the level you want.
That's not correct. Campden does NOT kill yeast. We winemakers use it all the time, and it doesn't kill wine yeast. Wine yeast is very sulfite-tolerant. You'd have to use such a mega dose to kill the yeast that the wine would be undrinkable.
My experience with stopping active fermentation is not positive. As TipsyDragon mentioned, most wine makers let it ferment out. Rack into sorbate and sulfites (sorbate works better in the presence of sulfites) to keep the yeast from reproducing. After a few days, sweeten the wine to taste.
The other common way to have a sweeter wine is to simply add sugar until the yeast are overwhelmed by the ABV and stop working. This can be tricky, though- with a happy wine yeast, you could get an ABV of 18% or so! Then you'd have sweet rocket fuel.