A bug in Outlook causes a delegate who has been removed from a shared Calendar to continue to receive Calendar notifications – even though he does not appear on the delegate list or in any Rules & Alerts.
This is because the rule in invisible – so you cannot see it in Outlook.
Resolution
Note that this deletes all delegates so you will need to add them again. Also note that some say you can delete all delegates and re-add them and the invisible delegate goes away too (which does not help if you have no delegates and the delegate still receives Calendar alerts!
Instead we need to open the mailbox using MFCMAPI to see the rule.
Download MFCMAPI here and extract the file onto a computer that also has Outlook installed (it will use the Outlook profile to logon to Exchange). Note that the 64bit & 32bit versions relate to the Outlook version not the OS. Works with Outlook 2010, 2013 & 2016. Just extract to a temp folder and run from there (it does not need installing)
After launching MFCMAPI click the Session menu and choose Logon.
If you are on the computer with the mailbox that sends the invites to the invisible delegate then just double click the Mailbox
If not, after logging on choose MDB, Open other mailboxes, then From GAL
Choose the suspect mailbox from the GAL, in this case Ana Williams. Click OK at the “CreateStoreEntryID flags” dialog that appears.
Navigate the Root Container down to Top of Information Store and then Inbox. Right-click Inbox and choose Other tables and then Rules table.
Depending on the number of regular inbox rules the mailbox has you may see more than one entry. To locate the invisible rule that handles email forwarding for delegates look for the rule that has a blank “Rule Name“, and has a PR_RULE_PROVIDER value of “Schedule+ EMS Interface“.
Before proceeding to the next step be aware that this process removes the email forwarding for all delegates on the mailbox. So before you delete it make sure you’ve made a note of the delegates who are supposed to remain on the mailbox, as they will need to be re-added.
Right-click the rule and choose Delete.
The final step is to re-add any delegates to the mailbox that are still wanted.
Thanks again to Paul Cunningham!
https://practical365.com/exchange-server/deleted-delegates-still-receive-meeting-invites-for-other-mailbox-users/