Potassium sorbate WILL NOT stop fermentation. Sorbate keeps yeast from reproducing. An active fermentation contains billions of yeast cells. Keeping them from reproducing has minimal impact. You can stop fermentation by chilling the cider, which will make the yeast dormant.
Also, Zeigler's Cider in my neck of the woods contains sorbate, which would've made fermentation difficult, if not impossible. Yours didn't contain sorbate?