My solution would be for SE to employ a few of the players in each server to act as chat moderators. Do not allow other players to be able to distinguish them in any way from other players when viewing the search list so that RMT will continue to spam them as though they were normal players. Allow these players to simply right click on a players chat, and mute them. It could be as simple as a 24 hour mute, but this would make a massive difference in the amount of spam that people are sent each day.

Some of those players may try to abuse that power though by muting people that have angered them in some way, and each time a player is muted, they may file a complaint against the moderator that goes directly to a GM. After reviewed by a GM, the player can either be cleared of the mute status, or left on mute status. A player that continuously has the mute status they put on other players can have their moderator position revoked for abuse.