You can delete a wall by from the "Overview" page ( you can confirm you're there by looking at the menu on the left of the page—the Overview item will be orange) of your wall. On the right side of that page is a button that has three dots on it, and if you click it options for that wall will appear. Click on the "Delete Wall" option, and then confirm that you really want to delete it. You cannot delete a wall if you've previously purchased that wall.