Truth is, Campden tablets are not designed to stop a fermentation and never have been. Using them for that purpose can get you into all kinds of trouble. There is really no ingredient that can be safely used by itself to assuredly stop a fermentation.
Campden tablets (potassium or sodium metabisulfite) are a sulfur-based product that is used primarily to sterilize wine, cider and in beer making to kill bacteria and to inhibit the growth of most wild yeast: this product is also used to eliminate both free chlorine and the more stable form, chloramine, from water