Customers would like to have an API endpoint that would allow them to revoke a portal-only customers' access to their sites, as described here:
https://support.atlassian.com/user-management/docs/revoke-access-for-portal-only-customers/

At the moment there is only Create a Customer is available via the API:
https://developer.atlassian.com/cloud/jira/service-desk/rest/api-group-customer/#api-group-customer