This is a great idea.

Concerns of abuse are overblown for two reasons:
1. This should be a chat log based option like blacklist so people can't just use it on someone out of the blue.
2. Players who abuse this can be subject to disciplinary action. This is not an anonymous report. It has who submitted it and what the actual text of the message linked was. When abuse occurs, there is recourse available and solid evidence.

That said, I think the first triggered action should be a temporary muting for all forms of communication to anyone not on friends list and a restriction on sending (but not receiving) friend requests. GM gets prompted to review and can swing the ban hammer. Should only take a matter of seconds for them to determine if it is RMT or not. They could also automate the ban hammer but make it take multiple instances of triggering the threshold within a certain amount of time.