Hearsay Heresy

This seems like quite a conundrum here. I think, though, that once we strip the situation down to its elements, the path becomes clear. First, online forums are public access. Just because you can link information to a persona, the source is still the forum rather than the one posting on the forum. Information contained in these threads is similar to that typically encountered eavesdropping in a public facility (park, cafeteria, shopping mall). The difference being that the banter is more focused with not as many different topics being discussed (although the number of conversations taking place can be similar). You certainly wouldn’t quote eavesdropped material, would you? I mean just because you can link information to a persona, the source is still the forum rather than the one posting on the forum.
So, you can’t quote the forums in your story, but the accusations are still there. This is what we would call a scoop. The story still exists. Someone must certainly feel something about these accusations. What about others that that read the forums? You could try to contact them, and see how they feel. You might try contacting the accused, allowing him a public forum to respond.
Regardless of the angle you decide to pursue, it is important to be sensitive in regards to the name issue. Slander and libel do to credibility what heat does to ice. Because the accusations may turn out to be slanderous, it may be wise to avoid getting caught up in the aftermath. It isn’t really necessary, in this case, to risk your credibility. Not while there are other avenues.
You could, however, coyly reveal the identity of the accused by getting them on the record. He can either deny the accusations or substantiate their validity by accepting or refusing to talk to you. This could be the difference between a lawsuit and a Pulitzer – albeit not quite so dramatic.
Zero or hero is question that could make anyone’s head spin. And with that, we’ve come full circle.