CSF Messenger offers a TEXT and HTML page to be displayed when a person is blocked in the firewall as well as an image to be displayed. Those can be found at /etc/csf/messenger and can be changed, but CSF will need to be restarted when changed.
Upgrade CSF in the server to the latest version from your WHM if available.
In command line or WHM CSF plug-in open /etc/csf/csf.conf
Search for “MESSENGER” and change it to MESSENGER = “1”
In command line add a user using the command useradd csf -s /bin/false
Restart CSF and lfd.
Confirm this is working by having someone give their IP, block them, restart the CSF and have them go to a page on the server.
Files to edit are located here;
README documentation is here: http://www.configserver.com/free/csf/readme.txt