Every time I get whispered by someone trying to sell gold, my immediate response is to type /ignore [name]. It may not be possible with the vanilla client, but later versions of WoW had Ignore in the list of commands that pop up when you click on the name of the user that chatted you. If that could be added, that would be fantastic.
Related to this, I've now discovered there is a limit to how many people I can ignore. (An extremely sad limit of only 20 in fact.) It seems to me that the limit (if it can't be removed) should at least be vastly increased. I'm now in the situation where I get whispered by gold sellers repeatedly and there is nothing I can do about it anymore.