In the vast majority of cases, only the internet provider who owns the IP can tell you which individual user was using it. You'll also need the date/time (and timezone!) of the problem, the ISP can then put two and two together and figure out which individual account it was. You can find out the ISP that owns the IP address by going to ARIN.NET and doing a whois on the IP. It will give you the name and contact information for the ISP in question.
It is unlikely that they'll release any of that information to you though, although they will often respond to complaints by threatening the user with a disconnect themselves if it is a legitimate complaint.
If the person atacking you is violating the law as well, you can report it to the authorities which can then subpoena the info from the ISP and go after the attacker... But in the vast majority of cases it's not going to be worth it.
Depending on how your guestbook works (is it a closes system, or some PHP guestbook you have the source for?) you may have some other options:
you could change the code for the guestbook to deny connections from the IP (range) in question.
It is unlikely that they'll release any of that information to you though, although they will often respond to complaints by threatening the user with a disconnect themselves if it is a legitimate complaint.
If the person atacking you is violating the law as well, you can report it to the authorities which can then subpoena the info from the ISP and go after the attacker... But in the vast majority of cases it's not going to be worth it.
Depending on how your guestbook works (is it a closes system, or some PHP guestbook you have the source for?) you may have some other options:
you could change the code for the guestbook to deny connections from the IP (range) in question.