I posted this on the goldspammer thread but I for one think it's worth consideration as its own thing.
Please add user-configurable regular-expression based filtering to all chat modes, skipping regex checks for players already on the friends list, and ignoring all chat from blacklisted players as already implemented.
This would be trivial to implement as far as coding is concerned since regular expression handling is part of most of the programming languages on Earth. The only challenge would be in implementing the UI for configuring it, and all that requires is a textbox with a "save changes" button that drops its contents into a string array to feed into a basic regex parser.
If any form of incoming chat text that isn't coming from a befriended player (which skips the regex and passes text unfiltered) or blacklisted player (which blocks the text regardless of content) matches any pattern in the box, it's dropped by the client and never displayed. Default it to no patterns slash pass everything. Naturally this would need to filter /say, /yell, /shout, and /tell text in order to be maximally useful, and probably should ignore /party chat as mechanisms already exist for dealing with mouthy party members.
Doing this would cause players to rapidly develop anti-RMT-spam filters and would effectively destroy the ability to spam RMT advertisements, as well as allow players to selectively filter out any form of chat they find obtrusive or annoying.