How can I delete a user, which does not appear anywhere else, from the "Customers" list?

[arben@contract.fit]

User is only in Customers list within a particular project. The user cannot be found in the section People within the project, nor on admin level.

[Jack Brickey · Accepted solution]

Are you wanting to "delete" the customer altogether or simply remove from project?

Delete Account or Revoke access - (you need to be site admin) Jira settings > user management > Jira service desk > search for customer, click on ellipses at the right and Delete account OR what I prefer/recommend is to Revoke access.