The Sucuri Firewall has several layers of protection, so you don’t need to worry about blacklisting IP addresses. It will automatically blacklist offending IP addresses, if it detects continuous attacking requests.
However, if for any reason you want to blacklist an IP address from even reaching your website, you can do the following:
Click here and enter the IP address you want to blacklist under the Blacklist IP Addresses section.
Click on Blacklist to blacklist the IP address once you have typed it into the box.
Keep in mind that depending on the situation, the Protected Page feature will be a better solution.