theymos, I do not think this scammer mark is such a good idea. This makes operators of the forum VERY vulnerable to real world libel lawsuits. I strongly suggest to reconsider this practice. Better stay away from this.
I think he'd need to prove that he has never scammed someone in his life, which is almost impossible.
You are wrong. The party accused of libel would have to prove that he is a scammer. Proof of truthfulness of a statement is a great defence in a libel case. However burden of proof lies with the party which is accused of libel. This of course varies in details depending on jurisdiction but generally this is the case. Libel litigation is quite a can of worms and due to general bias in favour claimants/plaintiffs and difficulty of defending such cases there are plenty of legal sharks doing libel cases pro bono.
Moreover, a simple letter to host's ISP with libel accusation will promptly generate some action which would force either prompt removal of allegedly libelling statement from a website or it's shut-down (ISPs generally have a better grasp on relevant law than "he'd need to prove that he has never scammed someone in his life").
I personally would chose to stay away from all this and keep neutral position in all those disputes. Much safer that way.