On one hand, I say that zoom abilities should be disabled, since if a user needs to zoom in or out, there's probably a different reason for it (text is too small, images aren't good) and the problem lies elsewhere (your RWD isn't properly formatted). On the other hand: to use zoom or not is a decision made by the user, it should not be forced by the designer. It's a standard capability and something that users are frequently using when viewing other websites that aren't properly formatted for that viewport.