The best way to deal with liability is to not allow grievances to be discussed period, but obviously that doesn't help people make informed choices.
I used to have faith that as long as what was said was true, or that there was a reasonable belief that it was true, the poster and the forums would be blameless, but a certain "jailhouse lawyer" proved that the courts can be used to legally harass people into silence without ever having to address the actual merits of the case.
In my non-expert opinion, the best way to deal with bad merchants is to educate people as to how to go after them in a manner that will achieve results, unfortunately, due to the variances in peoples standards as to what is fair and reasonable, public discussion of them isn't all that effective. I've seen unreasonable people post before even trying to work it out, and I've seen merchants including sponsors viciously attack people with legitimate compaints.
One thing that might be a solution is to require the users of this forum to agree to hold the forum owner blameless as a condition of use. But I'm not sure if it could be done in a legally binding way.
However, education is always the answer, posts such as this are a good start. It only takes one or two customers with a legitimate complaint following the right procedures to get a bad merchant in a big mess with people and agencies they don't want on their backs.
In reality, publicly posted complaints on forums don't seem to affect merchants all that much where it counts, even when the complaints are founded. And I've seen customers sit back and take it up the tail pipe when they should be standing up for themselves. Why anyone would let a merchant give them the excuses and run around I've seen some engage in, I can only think that the customer really doesn't know he has viable avenues of recourse.
Knowledge is indeed power, and consumers really do have the power to fight back, even without David Horowitz (which dates me if you know who David Horowitz is)