Unfortunately, I don't believe that there is any particular way to prevent Botnet attack. An article I read
here says that your only hope is survival. Unfortunately, even though your firewall blocks the traffic, the packet have still arrived at the router and thus saturated your bandwidth. If this is a continuing problem, you could ask your ISP to change your IP address. If they are coming at it by resolving a DNS entry, is changing your host name a possibility?