Yeah, those are ugly messages, but they are just that. Words with nothing to back them.
One of the drawbacks to immutability is immutability. I don't know that there is anything to do here besides ignore it.
I recommend you bask in the joy that this person is sending your money, ignore their messages and don't respond. As with all trolling or bullying if no one reacts it fades away.
Sorry you are dealing with this, it is pretty rank and rude.
RE: Steem Is Getting Ridiculous With Spammers and Is Causing It To Be A Unpleasant Experience. This message is for all witnesses...