To trace the IP you will need to do it on the fly as someone visits your site or something, but it's not always possible as there are different types of proxies, transparent type ones are the easiest to decode, I guess. If you just have an IP, then you don't have many possibilities, because proxies usually die in an hour timespan and there's no way do anything, and really even if the proxy was still alive, you would need to get in touch with the owner of that IP and in the end it's just not worth to bother...
