I added one customer into Jira and later removed it. Now, I'm unable to remove it from Jira permanently.
I see the customer is removed but still, I can see him I unable to remove the customer permanently.
You can always remove a customer from your Admin hub, go to https://<yourinstance>.atlassian.net/admin under (User Management > Jira Service Desk) search for the email of that user and remove the user. that should remove the user entirely from any Projects you have on your Instance. If you've done that and still see the customer, most probably the customer is a Jira user and you have to remove the user from the People's page of your Project Settings.