I have an idea... Do what EVE Online does. Allow players to set a monetary sum that must be paid before a tell can be sent to them from anyone not in their known circles. IE Linkshells/FC/friends list. It takes from the whole "My time is money" saying... Trust me when i tell you that if you take enough of their gil away just to advertise, then it would be in their best interest to find way outside of the game to do so