If a case has been discharged/closed and you are receiving notifications for tasks that are due after the discharge date, this is due to a setting not being configured on those activities or reports.
To prevent repeat due dates for closed cases , go to the settings of a specific activity or report and make sure that “Delete if due after close date and case closed (stop repeating)” is checked:
Once this change is made, a system administrator will need to delete any of the Due activities/reports that appeared after the close date.