To delete a pie, you must reduce its value to 0. To do so, use the Withdraw funds option to sell all investments in the pie and withdraw the resulting value. Alternatively, you can use the 'Export investments' option to remove all investments you want to keep outside the pie.
Then scroll to the bottom of the pie's 'Overview' screen and tap on 'Pie options':
Then tap on the 'Delete' option and confirm.
Note: Deleting a pie is a permanent action and it can't be reverted.