I'm affraid that DDOS is a common foe, and its hard to block because its like any other traffic. All you can do is make a good firewall so they can't portscan you to check for ports open.
http://www.iptablesrocks.org/examples/ has a good firewall for web servers. or you could get managed to host a firewall for you, but i doubt they will.
Hope this helps.