As I understand it, you should be able to target your legal action at the owner of the website as it is their responsibility to moderate content and they are the ones displaying the libellous information. It is then up to them to counter-sue the offending member of the discussion forum, for whom they should have contact details from the initial registration (more fool them if they didn't).
It is worth noting that the legal system is dependant on the location of the hosting server, i.e., if the server is hosted in the US, then US law applies.
Your first task would be to run a "whois" query on the offending website address to discover the registered owner of that domain.