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.
Files to edit are located here;
/etc/csf/messenger/index.html
/etc/csf/messenger/index.text
README documentation is here: http://www.configserver.com/free/csf/readme.txt