Here are the exact steps in case anyone else need help with it:

1) Upgrade CSF in the server to the latest version from your WHM.
2) Open /etc/csf/csf.conf
3) search for "MESSENGER" and change it to MESSENGER = "1"
4) In command line add a user using the command useradd csf -s /bin/false
5) restart csf and lfd. You are done :-)

If you want to customize the blocked page, you can change the index.html page located at /etc/csf/messenger


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