In some cases, with the Windows Outlook client, calendars that have been removed still send event reminders. This is due to the Outlook client treating the calendars in the “Deleted Items” folder as live calendars.

To resolve this, open the “Deleted Items” folder from Outlook. If no subfolders are shown use the Ctrl + 6 keyboard shortcut to switch to the Folder View.

You will be able to see the deleted calendars here, which you can now permanently delete.