Don't think the ISP can help you. Unless you can show the attacker is on the same ISP as you, can't do anything. And even then, its only their responsibility to stop the attacker, no so much stop him from attacking you. Your best bet is to change your IP. If your using DHCP just change the MAC address. Go to Setup > Internet > Manual Internet Connection Setup > Click "Clone your PC's Mac Address" then change it. Doesn't matter what you change it to, as long as its different, then save the settings, reboot the modem, then reboot the router after the modem comes up. If your on static, or need to keep your IP for some reason, try the suggestions below, but it probably won't get rid of the problem all together and some of them can cause other issues on your network.
To turn off Ping go Advanced > Advanced Network > Uncheck "Enable WAN Ping Respond" but by default it will be off. That will only block ICMP traffic though.
You also want to set NAT Endpoint Filtering to Port and Address Restricted. That will help with SYN overloads, but really there are so many kinds of DDOS attacks that the simple firewall you have on any store bought router will give you mediocre protection at best. Another suggestion, while your on that screen, is to turn off UPnP. There are exploits in UPnP that a attacker can take advantage.
Last, if you want to get fancy, you need to to Status > Internet Sessions. Try to turn off all computers except the one your on, or reduce traffic over the network as mush as possible, this will allow you to narrow down what connection is the threat. If you see a Internet address that keeps throwing traffic at a port your not using, likly thats the IP of the attacker. You can use that IP to get info about the attacker
here and
here.
What you need is a firewall with a inbound rule. I thought "Inbound Filter" did just that, but that seems to be just a white list/black list function for other features like Access Control and Virtual Server. Might be some way to rig it to work, but I would need more then the basic emulator I have to troubleshoot with. Good luck.