From : Blog Entry>>ILTA’s Blog Entry
It might be time to “let it go”. In Outlook, you receive pop-up reminders for upcoming and past events based on your calendar.  These reminders can build up over time if they are not cleared.  You have the option to clear ALL past-due reminders, if you want.   In Outlook go to File – then Options. In the left pane of the Options window, click on   Advanced.   Then in the right pane for the Advanced Options, click on the box beside   “Automatically dismiss reminders for past calendar events [More]