As this isn't a case of harassment, and is indeed a simple disagreement between two parties. It doesn't really warrant a report. As such no offical action will be taken.
I will however provide some advice.
It seems like you have somewhat attempted to make peace with each other in the past.
However it seems you two are incapable of coming to a worthwhile agreement, so i'd recommend not talking to each other. It doesn't seem that people have found nightmare to be a general nuisance its more of a problem just between you two.
And when I say don't talk to each other, that doesn't mean when you see him you yell out "OMG NIGHTMARE I AM NOT GOING TO TALK TO YOU ANYMORE!!!!11!!" As all this will do is provoke a response. Just don't talk, and treat him as you would any new player that you don't know very well.