The only way for sure is to use a verified email address. So you create a login to the message board and there would be a registration process. Then it would be verifiable (not Yahoo, Hotmail, etc).

Now, before everyone jumps, I am not advocating doing that, it would be a BIG pain for Greg to implement and I am all for privacy on the net.

Until then, its just like supply and demand. Demand goes down, price goes down. Ignore his posts and he/she will get bored and go bother Yahoo stock boards and the like.