This isn't actually a text format. This is one appearance setting or another specified in the Inspector. It might be a field border or the line specified for individual portal rows to separate one from another. Both can be made to disappear via the Appearance tab in the inspector. To make the row border disappear, select the portal while in layout mode select the appearance tab in the inspector and then select "portal row" from the upper drop down. The controls for the row border are found at the bottom of the "Graphic" section of this tab.