The Sucuri Firewall has several layers of protection, so you don't need to worry about blacklisting IP addresses. Sucuri Firewall will automatically blacklist offending IP address, if it detects continuous attacking requests.
However, if for any reason you want to blacklist an IP addresses from even reaching your website, you can do this by 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.