You can always try your local Police or cybercrimes division. If they can't help, they will refer you to who can work with you on the libel part. When you say that you suspect an imposter on the same IP address, are you referring to a public network such as hotspot you were at, or they are using the same IP address a.k.a. coming from the same network to harass you?
Either way, digging info up on them through whois.com gives contact info for their admin, mostly likely the webmaster or person in charge of the site, but not legal.
ONLINE CONNECTIONS INC.
Online Connections Inc.
5722 South Flamingo Road
Cooper City, Florida 33330
Also, reverse DNS'ing their site shows it is hosted in St. Louis MO. http://hostcabi.net/...o/188.8.131.52
And this one gives some pretty enlightening info about them: http://sameid.net/ip/184.108.40.206/
In all honesty though - if you are wanting to build a case against them for libel, you really need to have a good case against them, and a lawyer will advise you on that one. But something like this, might be too trivial to pursue for them unless they can find more instances against you or other people as well.