Just a few corrections from a REAL lawyer.  :)

First, the proper term is "defamation" 
and "libel" (for print/electronic media).  
Second, if I were you, I would request animanga 
to supply you with the IP address, so that YOU 
could track down the poster and sue HIM (if he's 
in the U.S.; unlikely, imo, but try anyway).  So 
be sure to save this thread/posts by the 
individual as evidence for a lawsuit.  :)