What should forum policy be on defamatory posts?

At the risk of playing the Captain Obvious card, might a better alternative be, not to take any of this entirely seriously?
Put on your thickest skin suit and have fun.

[Laird]

At the very least, best practices suggest that moderation be neutral and that a moderator separates their role as participant vs. moderator. On other forums, when a moderator is one of the participants in a heated discussion, they are explicitly excluded from acting unilaterally. Perhaps that could be a minimal suggestion (rather than singling anyone out) - if you are in a contentious discussion and you feel moderation is necessary, it should be passed on to a moderator who is not involved. 

Linda

I think this is pretty reasonable and sensible. There might be exceptions - for example if quick action is needed and none of the other moderators are around - but otherwise I don't have a problem with it. (With apologies to Oleo for such a serious response!)