I don't use shorewall, so can't tell you specifically. I am using iptables however, so you should be blocking all inbound traffic unless you need access on those ports. You don't list your inbound requirements. You have squid and postfix, so do these need to be accessible to the internet? If yes, then secure squid so that nobody can use it unless you authorise them to, and the same with postfix:
http://www.mailradar.com/openrelay/
that page will help you test and tell you what you need to do and fix postfix. As for the rest, you need to generate iptables or shorewall rules to block what you don't want access to. If you want to block on that particular IP:
iptables -A INPUT -d 216.66.15.109 -j DROP
assuming that 216.66.15.109 is your public IP address assigned to this server. If not, replace it with your public IP address. Because the one above that you mentioned is Fremont, California, and your IP posting here is South Africa. So change that destination IP appropriately.
will be suffice, and allow you to continue using your server locally. However it won't be accessible now from the internet for any of your resources. If you need access, then generate appropriate iptables rules prior to this to grant access for particular source IP's, or secure squid so that only authorised users can use it.
Based on the text from the cbl, someone is using your proxy to hide their conficker requirements, and so you were correctly blocked.