You can go to

ACP > Users & Groups > Find/Edit/Susend User

Search for "Guest"

You can now change the name of the Guest account to anything you want. What will happen is that in empty forums, the "By:" line will now contain the changed name.

It occurs to me that this could be quite useful if you empty your forums periodically. While they are empty, you could change the Guest account name to leave a short message like:"reopens_on_date" or "ComingSoon" or "ClosedForRevamp", something like that.

Or, if you have guest posting, you could have fun with the Guest account that way.