Actually, If people call him a fecker, an a-hole, or anything else there may be limited scope in which he can sue. Certainly in the US such comments are considered by the legal system to be personal opinion and as such not defamatory. Its why the recent Penn & Teller series "Bullshit" can call people by those references. So if the defamatory system bares any resemblance in the UK he may not have a legal leg to stand on.
Edit>> The other part of defamation law normally requires the offended party to prove their character was besmirched in some way. In a case such as a forum all the defendant (the forum operator) has to do is convince the judge that the comment was not their opinion and in any case the readership of the forum is a tiny percentage of the UK population so any such damage to character would be tiny.
Also (again), these sorts of threats are normally just legal bullying. If you write a letter to his lawyers explaining that all opinions belong to their posters, that you have no intention of censoring the actions of others and reducing their freedom of speech, and that should they pursue a legal case you will merely pull down the website replacing it with a notice that "this popular community forum has been closed down due to Frederic Shepherd's legal actions. Please contact <list reporters email addresses> if you are unhappy with this," they will often go away. Remember, as long as only letters/emails are being exchanged it isn't costing the forum admin anything, its only hitting Fred's pocket.