All reseller accounts are setup with a folder named suspended.page in your public_html directory.
This folder contains the page your client will see if you suspend their account for any reason (e.g. Non-payment or Violation of your business agreement).
This page can be customized to suit your business image. In order to edit it, download the index.html file contained in this folder, modify it with your favorite HTML editor (eg: FrontPage or Dreamweaver) and reupload it to the same folder.
Alternatively, you can edit the page's source code directly by logging into WHM and clicking on Modify Suspended Account Page under the Account Functions menu.