How do you not bring him to act 3? Do you just kill him? Because there's no dialogue to refuse him joining, and if you just leave the convo as he proposes to join, he just shows up at my camp anyway.
Their half assed attempt with him in act 3 just seems like a waste of time and money too. And in act 2 they could have had Jaheira do the shadow curse quest. Like when you really think about it, they could remove Halsin from the game entirely and it wouldn't make much difference. Sorry to Halsin fans maybe reading this, but it's kinda true, he's pretty irrelevant tbh.