To go along with the right-click report feature, SE needs to have someone watching and responding to those reports in real time. It won't work if they gather up a week's worth of reports, or even a day's worth, and then go through them. The spam would disappear if they combined three things:
(1) A rule that once you send a /tell, you cannot send another for a few seconds. Even a 3 or 4 second delay, too short to interfere with a legitimate /tell conversation, would be enough to ensure spammers aren't whispering more than 15 or 20 players per minute. Or, if they made it specific to restricting when they could next send a /tell to a different player, perhaps they could expand it to 5 or 6 seconds apart, so a spammer couldn't send their ads to more than 10 or 12 players per minute.
(2) Add the right-click "report RMT spam" feature to the chat log that we've been asking for for years now. (I'd also like to see "report harassment" both on the chat log and when interacting in person with another character, and a "report suspected bot" on character interaction, but the "report RMT spam" in the chat log is the one relevant to this topic, and the one that's most desperately needed based on frequency of the problem.) It should automatically include all relevant details (who sent the report, who was reported, and the text of the chat message reported) into the report, which should immediately pop up on the screen of whichever GM is assigned to handle these.
which brings us to:
(3) They need to have somebody on duty to handle these reports in real time as they come in. The reporting system then needs to be streamlined on the GM's end as well as on the players', so that the GM only has to glance at the report, see whether the message is indeed spam or not, and click on a button that says either "Yes, ToS violation RMT spam" (which immediately kicks out the offending spam bot and bans their account) or "No, unjustified report" (which just logs the fact that the reporting player sent a false report, so that later data mining can determine if anyone is abusing the system by repeatedly sending lots of false reports).
With this combination, spam bots could, for the most part, be banned within a minute or two (often within seconds) of sending their first /tell /say or /shout. Sure, occasionally some might last an extra 15 minutes or so if they start when the GM goes on his coffee break (assuming they'd only get someone to cover their position when they go on a longer lunch break). But thanks to (1) above even those spam bots lucky enough to get a few extra minutes would still only reach a tiny portion of the playerbase, and most would be gone after just their first few messages.
Not only would this get rid of the immediate spam bots, but the RMT companies would quickly find that this method of advertising is no longer cost effective. Spam bots that get banned after just a few messages are unlikely to find new buyers with those few messages, so will rarely bring in any income to justify the cost of setting them up. The current long-after-the-fact banning doesn't really accomplish anything because so long as the spam bots are bringing in money, then the RMT companies just keep replacing any bot that gets banned. But they won't keep replacing ones that are no longer effective. Soon after this immediate simple reporting and immediate simple banning goes into effect, the big RMT companies that have been plaguing the game will take their advertising elsewhere. Occasionally a newcomer to the market might try it out, and spread a few spam messages, but they wouldn't last long.
This in turn means that the job described in (3) of needing a GM to handle all these reports, is only a significant (likely full-time) responsibility for maybe the first week or so, only while clearing out all the spam bot accounts already in the game or the first few new ones created, before the RMT companies realize it doesn't work any more. After that, the number of spam bots, and therefore the number of reports, would drop off to an occasional trickle, and the GM handling them could spend the majority of his/her time on other responsibilities. (Of course, when a report does come in, they'd still need to have someone available to handle it immediately. As mentioned, newcomers to the market will try out spam from time to time, and it's important that whenever they do, they find it unproductive.)