To de-activate Customer Portal User, Goto Setup > Customer Portal Settings > Customer Portal Users. The list of Portal Users who have been added would be displayed. Select the user you intend to de-activate and click Deactivate Portal User.
Customers need to allow the following IPs in their firewalls or in SPF records to allow invitations/notiification mails from Zoho Support to reach their servers SMTP server public IPs: 1. 72.5.230.66 2. 72.5.230.67 ZOHO public IP: 1. 72.5.230.103