From the cPanel landing page, navigate to the 'Security' section. Now select the 'IP Blocker' button.
The main field 'Add an IP or Range' allows you to add new IP addresses to your Blacklist. Press the 'Add' button to confirm the address - be sure to conform with the allowable formats in the Note section below.
At the bottom of the page, you can review and modify your existing blocked IPs.
For the official resource on this topic, check out cPanel's documentation!