In one sense, how do else do you keep people away in a society with the right to roam freely? You make it unpalatable.
You could restrict tourist visas but then that impacts other parts of Italy and had to be done at a national level when this is a local problem. You could add conditions to the visas that preclude visiting Venice, assuming the national government went skiing with it, but now you need to enforce the movement of tourists within the country which seems difficult if not impossible. And that's not even considering visitors from within the EU who can travel freely anyway.
