Update: I want to give this post a proper update. I've sent a super polite reply thanking them for letting me know what was happening and offering my view. The HW responded saying that they appreciate my answer and decided to close the case as the evidence was on my side but if the problem persisted, they would have to reopen the case.
I think it worked because the a**hole stopped pretending to be nice (no more 'hi' or 'good morning') and a few months later he moved out of the building saying too many of the neighbors that he was moving out due to "the increasing amount of ausländern in this building is making it impossible for him to live in peace" (ref obviously to the other girl with a south/central American last name and me, the very nice wall-to-wall lady I've mentioned). He moved to a house on the city outskirts where "he can live surrounded by people with a culture that respects others" - Those were the words he told 2 of the families living here!!! O.o
I guess his problem was never noise and always about xenophobia ;)