When the customer receives a Customer Center Access email notification, there is a '<CompanyName> is powered by NetSuite' that appears on the footer. The footer is greyed-out and not editable.
To modify the message:
1. Navigate to Setup > Company > System Email Templates
2. Edit Standard Customer Center Access E-mail (or a custom customer center access email if they're using one) > Templates.
3. Remove "is powered by NetSuite - One System. No Limits."
Note: This can be replaced by a different message.