Answer:
Mass deletion of all your subscriber's contact details in the Letterhead website is NOT available to the user/administrator. The user/administrator must file a ticket to request it.
If the user/administrator wants to delete the contact details of their subscriber, they can manually do it by clicking on the three dots on the right side of the name's row, then hit Delete.
If you want to know more on how to manage your Audience page you may click this article.
Please be advised that once you DELETE the contact details of your subscriber, it will permanently remove from the channel, and NOT receive any emails or notifications.
To send your request, kindly email us at support@tryletterhead.com or by logging a ticket via the Help Center. Please provide the Channel's name and email address of the subscriber/s.