I add 1 crushed campden (potassium metabisulphite, or Kmeta) tablet per gallon at every racking and then at bottling time. (Or follow the recommended dosage amount on the package.)
If you plan to backsweeten, you also need to add 1 dose of potassium sorbate before backsweetening.
The Kmeta protects against oxidation and will also kill off any remaining yeast.
The potassium sorbate is birth control for yeast - it will keep it from reproducing, so in conjunction with the Kmeta from the campden tablet, it will stop any remaining yeast from restarting the fermentation process after you have backsweetened.
If you do not backsweeten, the sorbate isn't necessary.