Since I am running Mavericks (OS X 10.9) FontBook 5.0.1 may work a bit differently than your version, but I don't think Font Book has changed much over the last several years. In 5.0.1 the font to be deleted is selected by placing a checkmark in the check box to the left of the font name and then click on "Remove Checked" at the bottom of the Font Book window. At that point a verification dialogue should appear near the top of the window reading "Are you sure you want to delete checked fonts" and at the bottom right of that dialog are two buttons one labeled Cancel and the other labeled delete. Clicking the delete button should immediately remove the font.
