Well, I guess that they are already applying a pretty direct way to make these rude public of nowadays be punished: Do you know about the questions regarding the Silver Mace Quest or the Serpentine Tower Quest? The inhabitants of Tibia are quietly keeping for themselves the correct words and procedures to obtain it, making people believe that there is no big mystery circulating with the folk... Why would you share it to these shining armored arrogant people treating local population at cities as slaves and just asking "MISSION QUEST REWARD" around, sometimes even shouting, thinking that every shopkeeper or librarian is a particular crafted slave automaton? Why should they reveal fantastic treasures locations? Just to make the snob gentry of vocations more rich and even more alienated about basic social behavior?
Unless a NPC got an emergency, really important matter or big danger roaming around, they will still be quiet about anything else since it is always a chat where the adventurer wants more profit, no matter what. Unless the adventurers starts being more educated and nice to common population, we may never hear what the NPCs have to say about unthinkable places and chests. Keep in mind what an old sign says in Rookgaard: "Only the humble may touch the Sword of Fury This is the Sword of Fury". Maybe this legendary sword stills a dream after years due to the same reason of every other one obscure quest on Mainland: Lack of humility. And you can perceive on any store on Tibia.