Client did not pay?
Add opacity to your widgets and decrease it every day until their app completely fades away. Set a due date and customize the number of days you offer them until the app is fully vanished.
Faded( // Due date should be in yyyy-mm-dd format dueDate: '2021-01-15', daysDeadline: 12, child: MyApp(), )
And.... Faded! the app is all gone now..