I don't think it's necessary. First, people don't generally trust hearsay from unknown sources. Those sources have to prove themselves first before they're taken as genuine. That goes for anything, not just game development.
If Person A writes that Capcom will be announcing Onimusha U in 2014, few people are just going to take their word for it. If it turns out to be untrue, forum users will just police the forum themselves. If Person A returns to say that Valkyria Chronicles 4 will be a Microsoft exclusive, we'll all just say, "hey, what about Onimusha U?"
This stuff builds up. Liars won't be taken seriously and their lies will be remembered.
Now, if a person keeps making up rumors that prove over and over to be untrue, then I think that should count as trolling. That's in obvious cases, though, and after they've been warned.
