It's difficult to track it to an actual person, as it's unlikely that he owns the IP address himself. You can track down his ISP, and if it's a dedicated IP (meaning it doesn't change each time they login) you could complain to his ISP. If the IPs are close, you're looking for a needle in a haystack. If they have broken laws etc.... the ISP may be willing to track down the user on that IP during that exact time etc.... if they are just causing you trouble on your site and being an annoyance, it's unlikely they will get involved.
It's easy for users to get other IPs or use proxy servers... so generally banning by IP is pretty uneffective if the user has some PC saavy.
This page:
http://combat.uxn.com/Enter their IP into the ARIN section. That'll show you who owns the IP.