Game Server being DDoS'd, We know who it is. Can we report them somewhere?

Can you create a setting to deny requests from that IP, or block it? If you know who it is, and if they cause any damages and so forth you could call your local police. They could then launch an investigation, and track down the person especially if you have their public IP, if you're willing to press charges. Or another solution, I assume your game server is connected to your local ISP? Call your ISP and ask if they can block that IP address on their core routers, or the router that your service is connected to.