You don't stop druids from doing it, anymore than you stop a wizard from blasting up a tavern with fireball, or a rogue from trying to steal the crown from the king's head in the middle of the throne-room, or the fighter from hacking up a dozen bar-maids. Rather, any PC that does these things is going to have major in-campaign consequences, and NPCs that do it define themselves as villains. And, likewise, players that insist on continually acting villainous and petty in my heroic games will eventually be discouraged from playing with us.