The WHMCS Firewall module contains a custom “You are blocked” page for its features that block access for various reasons.
One such feature is the “Account under review”. This feature allows a new user to sign-up for an initial product but can still block access to the clients section. It can also be used to moderate existing clients that are showing questionable behavior or are severely behind on their payments (for example) and you want to block server control access (Force them to pay) but do not want to suspend their server/product and possibly effect hundreds of their end users/clients.
The Blocked page that results is generic and can be customized to your liking.