Tasks in your calendar need to be marked complete for them to be removed from your list of unscheduled tasks. We recommend you do this while you are in the store, conducting your visit.
Open the task in your calendar and mark it complete in the sidebar. You may need to refresh the page, it will then appear crossed out.
If you are no longer doing to do a scheduled task, clear it from the calendar so it returns to your list of unscheduled tasks.
Open the task and scroll about halfway down. Click 'Clear schedule'.
If you don't intend to do a task at all, open the task details and click 'Dismiss'.