Anyone with access to Jeff's logs would be able to reconstruct who posts what. However, you would have to hack into is server, which (hopefully) is not trivial. I have no desire to try this.
An alternate approach would be to spearfish your way into Jeff's password, but I suspect that type of social engineering would fail. (if you could log in as Jeff, you would have his admin credentials).
Now, if someone really posted something bad, you could go through the court system...but that will be timely and costly.