I finally left the negative feedback because the user has publicly spread lies about my person several times in several threads. If this seems to be inappropiate I would like to know from other DT-Members how to deal with such a user.
Imho it's not the best way to use negative feedback as some sort of retaliation. If I am you, I would just leave him neutral feedback, put him on my distrust list and forget about him. No point even arguing much with him as he provided zero proof for his claims.
People are free to share the merit as they please as long as they are not selling them or sending to alt accounts and you provided absolutely no proof of either.