Be the bigger person, keep your feedback at neutral, and let the entire community see what a nutjob that guy is.
It is neutral and will remain like that.
(You may also want to block his PMs like I had to, or else you will get constant PMS offering to remove his negative feedback IF you leave him positive.)
I have ignored him as I'm done with this. I have put too much effort into genuinely trying to make peace with this user just to have them to back out of everything in an instant.
Lauda on the otherhand asked for a cryptocoin worth 0.25BTC for next to nothing, and threatened to damage his reputation if he did not comply. There is a very big difference between the two.
I am starting to think that a) You are doing this on purpose; b) You have comprehension issues. The coin was not worth "0.25 BTC" unless you can see in the future(?). Even now, it is arguable if it is worth 0.25 BTC. Similarly if I offer someone ~$670 for 1 BTC now, you can't claim I offered "next to nothing" if it reaches a price of, e.g. $1k after X time.
Lutpin - I see you left him needed feedback, and he turned around and left you negative instantly.
I even explained to him several times, during our negations, why bad retaliatory feedback should not be left.