Use a Browser developer tool like Firebug for this sort of CSS work. They’re imperative to debugging CSS.
I used Google Chrome’s built-in Developer Tool and easily exposed the HTML and CSS structure of your webpage http://snag.gy/rha82.jpg . As you can see, there are current selectors of that section visible on the right-hand side.
You ought to therefore use .author-info to select that Author section of the page with CSS. Then use your display: none; style.
All CSS modifications should be either within the Custom CSS section of the dashboard in Twenty Twelve, or a Child Theme’s stylesheet.