Exterminate the Vermin

On top of the fury of the overseer, and this "key" not being here, we have a pest problem. Sneaky little vulperan thieves keep ransacking our site. We have taken some prisoner. We have them around the upper level of the dig site, working off their misdeeds. However, I believe a more permanent solution would be preferred. Please, oh mighty cobra, give us the honor of getting rid of our prisoners with your most potent venom!