There's always an "Abuse" email address on the whois of a netblock for reporting misuse of an IP address.
You can use http://whois.domaintools.com/ to do a whois lookup to get the address.
Is it worth your time? That's your call. Will it lead to anything? Nothing you'll ever see. But many of the sites I fix come from people who were first alerted of the problems on their server by someone sending the hosting company an "abuse" notification email. So it definitely can make a difference.
Note that the IP address you track down is almost never the attacker himself but rather a hacked server or computer that he's using as a relay. So keep that in mind. You're not alerting the authorities on the whereabouts of a miscreant; you're notifying someone that his computer has been compromised.
EDIT TO ADD
You give the information you have to the appropriate authority, and then you're done. That it. As a rule, hosting companies will not share personal information of their clients unless you are local law enforcement with the appropriate warrant or court order. It's their liability if they do otherwise.
Don't expect a follow-up report from them, don't expect names or arrests or anything more than an acknowledgement that they heard you -- sometimes not even that. These companies often deal with dozens of these reports a week or more. Their abuse team will deal with it, and they appreciate your assistance as they want to keep their network clean, and your report will probably trigger several days worth of activity. But they have a clear-cut policy that they follow to the letter for liability reasons, and it intentionally doesn't include reporting back the original reporter. Nothing against you specifically.
Also, remember that though you found the hacker, It's almost certainly not his account on the server.