Hi,
According to you post, I understand that Outlook still pop up calendar reminder, however this calendar has been deleted.
If I misunderstand your concern, please do not hesitate to let me know.
How about OWA?
If the issue only occur on Outlook client, please try to run Outlook.exe /cleanreminders to test. Also try Outlook.exe /resetfolders if previous command do not work.
If the issue persists, please download
MFCMAPI and follow below steps to check reminder:
1. On the Session menu, click Logon and Display Store Table.
2. If you are prompted to select a profile, click the profile name in the Profile Name list, and then click OK.
3. Double-click the mailbox that contains the inbox rules that you want to delete.
4. Expand Root Container, and then expand Finder.
5. Double click Reminders, we will find some items did in this folder.
Note: Outlook will not display any reminders if there are no item in the reminder folder. One item in the reminder folder mean an reminder should be displayed in Outlook.
Thanks