What's the point? If they're blocked then they can't reach your server anyway - game over.
Use the inbuilt functions of CSF, to firewall Class C netblocks and distributed attacks to the same email account.
Use csf.blocklist to block the most common idiots, plus add a few custom ones.
Turn off the CSF email block alert, if it bugs you (which is what I do) and just let the firewall do its' stuff. If any of your clients (if you have any) get blocked, then they'll soon let you know and you can remove their block.
[The only email alert that I set on is for console access - which should only be me. You might want to keep ssh access but that can be a pain if you sftp backups to the server, for example.]