As of Inquisition, we have seen 4 possessed Wardens throughout all of Dragon Age media:Sophia Dryden
: alive when possessed, became a ghoulKristoff
: dead when possessed, became undead, spirit not corruptedAnders
: alive, spirit and mage merged, spirit became ‘corrupted’Fiona
: alive, possessed, demon was evicted, apparently cured of the taint
So here’s the thing. In canon the Warden has run off seeking the cure for the Calling so they can live happily ever after, with the blessing of their LI should one be in the picture (or literally just with their LI, in the case of Zev). Fiona got the cure during the Calling before she conceived Alistair, and I refuse to believe it was Maric’s magical penis that cured her on principle.
brennacedria on tumblr proposed that spirit possession was the cure. But here we have Sophia and Anders, both of whom demonstrably were NOT cured simply by the mere act of possession. Here’s the thing though - they’ve still got the spirit as a rider, unlike Fiona. Given the evidence, I propose this is how the 'cure’ works - if a demon (or spirit - I’m going to use demon for convenience) possesses a Grey Warden, the taint transfers to the demon. If the demon is evicted, it will then take the taint with it. I suspect the actual mechanism of the taint is something like a magical retrovirus - the demon takes the magical part and after that happens the immune system can fight off the virus part on its own, given time and rest and the like (explaining why Fiona still shows physical symptoms at the end of the Calling according to people who've actually read the book - she’s still sick, just not Blighted).
If my hypothesis is correct, that means when Justice and Anders merged, Anders was 'cured’ of the taint - but Justice got tainted instead. Since they share a body and to at least some extent, a mind, that doesn’t help that much for anything. Look at this description of Justice's actions in the Anders short story:
Not the aftermath of a battle as I’ve known it, but a bloody abattoir of rent limbs and torn and eaten flesh.
This doesn’t sound much like some random demon - no demon I can recall eats flesh. But darkspawn certainly do.
The fact that Justice doesn’t show any corruption when possessing Kristoff’s corpse might have less to do with lack of Anders’ anger and more to do with Kristoff being dead at the time and therefore somehow unable to pass on the taint to his passenger. This could indicate something about the nature of the taint, possibly related to its inability to be harbored in nonliving material. It also reminds me of the effects of the Taint on the Evanuris, as described in the Arbor Wilds codex entry for Andruil.