1. Login at the Customer Control Panel with your main account's username and password.
2. Once logged in you'll see a link that says "Login via IP Address" under the Parallels Plesk Panel section at the bottom left hand side of the page.
3. Once you click on that link, a new window will come up, you might get a certificate warning, just click continue, then your Plesk Panel will come up.
Note: You can also log into your Plesk Control panel directly by the server's IP address (Example) "https://18.104.22.168:8443/plesk/modules/firewall/"
4. Go to "Tools & Settings", then go to "Security" section.
5. Click on the "Firewall" link, make sure your firewall is "Enabled".
6. Click the "Modify Plesk Firewall Rules".
7. Click on "Add Custom Rule".
8. In the "Sources" section, add the IP address you want to block in the "Add IP address or Network" form field.
9. Click the "Add" button, then OK.
10. Click the "Apply Changes" button.
11. Then click "Activate" to commit the rules update.