Deleting recurring meetings of a former employee

In order to delete recurring meetings of a former employee,  please refer to the conditions below and act accordingly,

Condition1: If the user is still licensed with Office 365 Exchange Online

You can use an Office 365 admin account to add the Full Access and Send As permission of the user mailbox to that admin account.

Then you can configure this admin account in Outlook client and the user's mailbox will automatically show under the admin mailbox. Just go to the user mailbox's calendar and find the meeting to cancel it, about how to cancel the meeting, please refer to this article: Cancel a meeting

Condition2: If the user's license has been removed

In-Office 365, the user's old email, contacts, and calendar are retained for 30 days. So in the 30 days, we would suggest you temporarily re-assign the Office 365 license to the user appearing under Users à Deleted Users in the admin portal, then convert it to a shared mailbox and remove the license. 

After that, you can add the Full Access and Send As permission of the shared mailbox to the admin account based on the same methods I mentioned above and access the shared mailbox to cancel the meeting.

Condition3: If the user's license has been removed and the 30-day retention period has gone.

After the user account is deleted, it's listed on the Deleted Users page in the Microsoft 365 admin center. It can be recovered within 30 days after being deleted. After 30 days, the user account and mailbox are permanently deleted and not recoverable.

Important: If you want to cancels meetings in the specified mailbox where the mailbox is the meeting organizer, and the meeting has one or more attendees or resources. Please refer to the cmdlet below,

Remove-CalendarEvents -Identity <mailbox email address> -CancelOrganizedMeetings -QueryWindowInDays 120

Example: Remove-CalendarEvents -Identity [e-mail] -CancelOrganizedMeetings -QueryWindowInDays 120

Note: The above example cancels every meeting in the mailbox [e-mail] that occurs on or 120 days after today's date.

Please refer to this link for more info on removing calendar events