This does sound like a notification scheme configuration as mentioned by Bushan. If you do not manage to find anything in your notification scheme that would explain why this ex-user is getting these notification, you should also look into the users/groups defined in the Project Roles related to the perticular project. If none of these options provide you with any insight on why the user receives the notification, I would advice you to change the email address of the user to a bogus address, to ensure he does not receive any notifications from that point on, and contact Atlassian Support to provide you assistance in the issue.
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