A label can be turned on or off without deleting it, by selecting the label in the central browser and pressing the Hide button. To turn it back on, select it again then press the Show button. Press the Delete button to delete it. This browser allows multiple selections, which, for example, allows you to delete several labels at once. To select everything in the current category, press Select All; to unselect them, press Unselect All. If nothing is selected, the action is applied to everything. Thus, one way to turn everything off is to press Unselect All then press Hide. (It may seem counterintuitive, but it was done this way so all the labels could be deleted by just pressing Delete.)