Are you looking to remove the customer from accessing a project or are you looking to completely remove the customer from your instance? If you just want to remove access then go into Project settings > People and remove the customer from there. If you want to completely remove the customer from the instance you need to have Org admin access and go to admin.atlassian.com > Products > Jira Service Management and either deactivate the user or delete them. Please note that if you delete the customer, any associated tickets will replace the customer name with Deleted user and you have no way to revert from that.
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