The reason a terminated employee is still appearing under Unsubmitted Timesheet is that, when terminating the employee, you have to make sure that you have removed him/her under the Mandatory Timesheet Policy.
You can check this by following the steps below:
Navigate to Configuration, Organisation Settings, Time & Attendance, Edit.
Locate the relevant employee's name and remove him/her and select Save.
This will then solve the issue, and the employee will no longer appear under the Unsubmitted Timesheet.