Closing a tab will prevent any further items from being added to it. When a tab is closed and paid, it will disappear from the "Summary View".
To close a tab, go to the "Summary" view, select the tab, and press the red "CLOSE" button at the top of the tab screen.
Then press "OK" at the "Close Tab" Prompt.
You will see the "CLOSE" button has changed to "TAB CLOSED"
When you're not viewing the tab, you will still be able to see it is closed as the tab name will be red in the list on the left hand pane.