The wily spammer can use many tricks to hide his true home address. There are some techniques to overcome some of the spammers ruses. Take a look at this example or this one
Now that we have an IP address to work with we can use one of several tools to get more information about the originator. We can plug the IP address into Ping and get the name lookup. You can use NSLookUp to do the same.
More usefully you can use Whois to look up the site owner's email address. lastly you may need to try Trace Route or TraceRt for really difficult cases.
See the tools page for more detailed information about using these tools. I generally have good response from messages to major ISPs about spammers on their sites.