A method I have for consideration to combat RMT spam entirely:
Instead of deleting RMT, you mute spammers so it appears to their client that they are actively sending out spam messages when in fact they only display to the client that is sending them. They continue to spam for hours and days without actually sending any messages. Allow this feature to function by perhaps 3 separate clients using a "report spam" function when receiving a spam message.

Another idea is to create and enable a function that allows you to block whisper messages by users of unverified accounts(which can be defined at a later time) people not in your linkshell(s), and people not in your free company.

Thank you for your consideration,
a 20 year Final Fantasy fan.