Tap the double arrows next to your portfolio balance to bring up the 'Portfolios' menu. In minimized layout, tap on the 3 dots in the right top corner and select 'Switch Portfolio' or 'Add New Portfolio' to bring up the 'Portfolios' menu.
Once the portfolios menu is open - tap 'Edit' and then on the red delete icon (red circle with white dash) to completely remove a portfolio. The app will ask for your confirmation before deleting. Please note that this action is irreversible!