If you use Microsoft Outlook on OS X with the Exchange calendar, you will be familiar with the "Office Reminders" dialog window that will pop up and remind you of calendar events.
The problem is that this service will continue to pop up for events, even if you have closed the reminder application. Although the application has been closed and cannot be seen in the Dock, Outlook is actually running some background processes that prevent the reminders dialog from closing.
These processes can be stopped if wished, unfortunately in a not so user friendly manner, as you have to manually kill the processes using the Activity Manager.
To open the Activity Manager, open spotlight (⌘ + Space) and start typing "Activity" until you see the application.
Once open, you can filter for running processes, type "Microsoft" in the search bar in the top right.
Finally once you can see these two processes "Microsoft Database Daemon" and "Microsoft Office Reminders", click on each one and click the "Force Quit" button which looks like a cross in a stop sign. Once the processes have ended, the pop up will no longer be able disturb you.