Can posts on moderated internet forums be held libelous?

Yes. Moderators are liable to maintain puriy of site as per law so on their failure to do so they can personally be sued.

It all depends on the topic, the way one looks at it, his/her personal inclination/affiliation OR absolute neutrality etc. In maths, 2 + 2 cannot be 6....... but such yardstick does not exist in societal aspects. I have personal experience of being a moderator myself. A billion population comes up with trillions and trillions of concepts, theories etc. While some of them are absolutely irrelevant, abusive, defective, misleading etc; quiet a percentage of inputs pose difficulty to judge. It is better not to miss a point than miss that specific point that would be 'The stitch that saves nine'. 

Punishing a moderator for politics will boomerang.

Thank you Suresh. I had written from the view point of a moderator being sued, and his or her need to be careful when allowing (as I stated antes) potentially libelous statements (not just statements or theories) about another person or other persons. The question of 'punishing' anyone was far from my mind. Such cases are notoriously difficult for the claimant to win. I wondered, loudly, IF it was worth for the moderator to get stressed out by being sued for allowing potentially libelous stuff on a moderated forum. A good case and point, stuff which is posted by readers on the major online Indian nespapers. 

Well, if it is moderation of online comments at a news item, the comment should not transgress/abuse individual/s rights causing defamation due to falsity and such well established ethics that which comes naturally for an editor. It is to ensure that one does not call the other vulgar names etc that which happens in verbal duels.

It is well for the editor/moderator to note that most often, one is not a first hand witness. In most of the situations, one depends on one's personal knowledge, judgment etc which are all ifs and buts. In most of the cases, tit bits never prove anything conclusively whatever be the level of analysis at the editorial desk. But, one piece of explosive material can spoil, rather KILL that target individual. There are various ways to kill. There are various ways to defame. That is why concept of libel got evolved.

Moral education teaches one to restrain from being bad, promoting bad. The three monkeys (shut eyes, mouth, ears) is worth recall. Editors/moderators are expected to practice, promote that impartially. That is all.

Very well stated, Suresh. My feelings are very similar.