Right-click on the Seating Chart page and select Seating Chart Properties. You can also press the "Other Properties" button on the Layout tab and select "Seating Chart Page".
On the bottom-half of the properties, on the left-hand side, Diagram Format should be selected. Underneath it you can select the number of columns. Enter 1.
Note that this is part of your Layout, so this setting will be saved with your layout template.