Well, I have never sucessfully stopped fermentation with campden. All you might do is oversulfite your wine.
If you really want to ensure no further fermentation (and you don't want to bottle carb), you wait until it's done and then add one crushed campden per gallon and some sorbate. (I think around 2/3 tsp or so per 5 gallons, but please check this- I'm going by memory here!) Wait a few days, and then you can sweeten it to taste. This is the only sucessful way I know to sweeten wines without restarting carbonation. Wine yeasts are generally not susceptible to campden, so I use campden all the time during fermentation, including at every other racking and at crushing the fruit. Sorbate doesn't kill yeasts, either, but in conjunction with campden, it can stop the yeast from reproducing, so that fermentation won't restart. Stopping fermentation in the middle of the fermentation period is like stopping a runaway train, though- so it's best to wait until it's done and then backsweeten.