To say nothing of the fact that, realistically, we only have Lucas's word that anyone was selling heroin out of these apartments at all, and seeing that he apparently hallucinates men picking up 12 girls, sexual blushing, faucets talking to him when they drip, women outside screaming that they want his babies, I think it's highly likely that no one in the building was selling drugs.
Lucas got kicked out because he threatened all of his neighbors, and left notes on their doors, possibly more than once, knowing how permissive this sort of place is likely to be.
Everyone speaks about his getting kicked out of the apartment as if threatening heroin dealers was the focus of the issue, but I don't think there WAS anyone selling heroin.
I think Lucas was just trying to get women from any of his neighbors.
It's pretty clear that Lucas doesn't understand the differences between various sorts of hard drugs at all, he just uses them as insults.
When he doesn't like someone, he says they do meth or heroin... just like he says they are Republican, or a bigot, or petit bourgeoisie, or an oligarch, etc etc etc.
It's just another word, that translated from Lucas to English, really means "You have something I want and you won't give it to me, so I'm going to say you're bad for having it".