These are internal confluence user accounts that are showing up on the customer list for a service project. They are not listed under "People" in project settings or "Portal Customers" in Atlassian admin.
Advertantely people who don't have access to Product but have Site access will become Customers and end up showing in the Customers. [ If "Anyone allowed on the customer access settings" option is chosen ]
Check the below option to remove them
[ Let me know if you have that option already enabled and still they are visible in the customers section)
Please take a look on this answer - https://community.atlassian.com/t5/Jira-Service-Management/How-can-we-stop-customer-registering-to-Service-Desk-with-quot/qaq-p/1432209