Some people find that they still get popup reminders for some events even after they have deleted some events from their Outlook calendar, or even after removing the entire calendar. The problem occurs with deleted and canceled meetings in the Outlook desktop client when you have mailboxes hosted on the Exchange Server or Microsoft 365.
When you delete events from your Outlook calendar, they are moved to the Deleted Items folder, and continue to send popup notifications (reminders). First, try deleting events from this folder.
Switch Outlook to Folder View and select the Deleted Items folder. If you have deleted a meeting or other calendar event, find it, right-click it, and delete it permanently. 
Deleted Outlook calendars are a special type of container that will also appear in the “Deleted Items” folder. In this calendar, you will also find events with reminders. Delete all the events in the calendar one by one, and then delete the calendar itself.
If there are any other calendars connected in the settings of your Outlook mail profile, delete them as well (Control Panel -> Mail -> Email accounts -> Internet Calendars tab).
It would help if you ran Outlook with the /cleanreminders parameter to clean up old reminders in the mailbox calendar:
- Close the running Outlook app;
- Click
Win + Rand run the command:outlook.exe /cleanreminders
Some of your calendar events and reminders cannot be deleted from Outlook. An error may occur when removing a meeting request:
This meeting cannot be removed because you are the meeting organizer.
Even if the original calendar event has been deleted, reminders for such an event may still appear. This indicates that the event reminder is corrupt.
In this case, try deleting the calendar item using the MFCMapi (Microsoft Exchange Server MAPI Editor).
- Download the MFCMapi tool (https://github.com/microsoft/mfcmapi/releases/);
- Expand Root-Mailbox -> Finder -> Reminders;
- Find a recurring meeting by sorting the list by the Subject column;
- Click on the meeting and select Delete Message;
- In some cases, also check the Deleted Items folder in MFCMapi. Find a recurring calendar reminder manually or with a search. Delete with the DELETE_HARD_DELETE option.
- Start Outlook with the option to clean up reminders:
outlook.exe /cleanreminders