I think it is very useful if there's at least 1 person in difficult child's life--someone whose response to him can't be simply ignored or trivialized, like a family member or close friend--who turns completely away from him in disgust and extinguishes the relationship. Here's why: difficult children get *very* accustomed to family members (especially parents) being unable to turn away from them, always forgiving them (or inclined to forgive), enabling them, etc. They exploit this as a perceived weakness. But if one of them--obviously, it can't be a parent, because a parent can only very rarely do it--finally announces "OK, I've had enough of this--this is intolerable" and extinguishes the relationship with the difficult child, that can be a real jolt.
I did this with my difficult child nephew. At his mother's appeal to me, I moved in and lived with her family (her and two sons, a younger easy child and older difficult child) for 15 months. My role was to protect her and easy child and their property from thieving, lying, drug-taking and -selling, marauding, hell-raising 17-18 year old difficult child. Along with playing "bouncer" and returning safety and sanity to their household, I teamed up with his mother to try to nudge him toward improvement, self-awareness, etc (he had weekly counseling with therapist too, when he deigned to go to it). As is usually the case with difficult children, the latter didn't work: I was able to stop the household thievery, bullying, extortion, menacing, etc via many, many violent encounters with difficult child, but he wouldn't change. I finally left in disgust 15 months later (disgust at his mother for never-ending enablement, refusal to throw him out of the house for the sake of the easy child, etc), but I made it very very clear to difficult child that I thought him the worst form of lowlife, a vile *******, and that I wanted never to see him again--in short, that I hated his guts and was utterly quit of him. He put up a tough front but he was clearly very shaken: an uncle who knew him very well, who had come in love and hope and support, now washing his hands of him forever in loathing and fury and contempt.
Parents can't pull this off. Siblings can't either, although I think they often covertly, tacitly hate the difficult child, quite justifiably. But I think it's a *good* thing when it can happen: it teaches the difficult child that he will NOT always be forgiven and given yet another chance, that it's possible that their hideous behavior can actually extinguish what had formerly been love--i.e., that they are snuffing out vital connections via their misconduct. That they can and will be hated due to their behavior, and that important people in their lives can and will turn away from them and never look back. They need to learn this.