I found another workaround if you want Halsin around for his role in the story in act 1 and 2, but don't want his creepy and disturbing presence in act 3:
After he rescues Thaniel from the Shadowfell, but before you re-unite Thaniel with Oliver, you can kill Halsin and he remains permanently dead and cannot be resurrected. But you will still lift the Shadow Curse after defeating Ketheric. My party is now happily moving along towards Baldur's Gate with Halsin somewhere in a crate at the Last Light Inn and he won't bother anyone in act 3.
