Right after she'd killed a fraternity. I'm thinking that is up there with killing Angel. Much as we care about Anya, as long as she is unrepentant and the people are dead, Buffy really has little choice but to treat her as an imminent threat. She was fulfilling wishes and doing it with deadly effect. Slayers can't allow wanton murder even if they care about the perpetrator.
My problem is Buffy's lack of consistancy. She goes straight to Slay mode with Anya and protects Spike who only shows remorse for trying to rape Buffy and shows no remorse for the other atrocities he committed against Buffy and everyone else. Passing time in the basement waiting for Buffy to come up with something for Spike to do, doesn't count as contrition.
Also, Spike's trigger is an imminent existential threat to the entire Slayer line that Buffy blows off. Just..... because.....
So..... if Buffy is going to default to the death penalty for Anya, she needs to default to the death penalty for Spike.
A serious question: Do Vengence Demons have human souls? I don't know and can't remember any time it was brought up. I've seen many posters claim that Anya "had a soul" when she committed her atrocities, so, killing her is righteous, but, I don't remember seeing vengence demon souls addressed on screen at all. As bad as the vampire soul canon is, the vengence demon soul cannon is 100,000 times worse.