Navigate to the Library via the Page Navigation Menus on the left, and select Brands.
Find the Brand you wish to deactivate and select it. A popup window will appear and in the bottom left hand corner of the popup you will see an Active button, select that to deactivate the selected Brand and then hit the Update button. The Brand is now deactivated.
If you wish to reactivate a Brand, from the Brands Library select the SHOW INACTIVE button on the right hand side of the screen. Now you will see any deactivated Brands in a light grey text. Select the Brand you wish to reactivate, and select the Inactive button in the bottom left hand corner of the popup window and hit the Update Button. Now the selected Brand will be Active and selectable again.