ip flood? like DOS of tcp syn packets or something?
Easiest way would be to install a packet capture tool, like etheral (or packetyzer, which is ethereal with a better frontend).
http://packetyzer.com I believe.
Once you get it installed and functional, just start sniffing packets on your network card. The originating IP address is contained in the header of every IP packet...of course, the ip could be spoofed and randomly change, in which case that information is pointless. But most people are idiots and don't know how to spoof their ip headers.
Once you have the ip, you can do a reverse-lookup on it and figure out who their provider is. It's even possible to figure out where they're physically located at, too, thanks to naming conventions of intermediate routers. That's kinda hit and miss, but possible.
Given the provider, you have someone to complain to. Other than complaining to the ISP (or counter hacking), you don't have much recourse after that, though.