Wolfsbane
(Info)
Wolfsbane has a variety of effects on werewolves, apparently depending on variety. Derek used it when burying Laura to keep her from shifting out of wolf form. When Scott was exposed to it in Stiles's car, he developed difficulty breathing and could not control his ability to shift.
Nordic blue monkshood, a rare wolfsbane variety, is able to kill a werewolf in 48 hours.