What would the purpose be long-term? Even if you stop it they can try again next action they get. You probably just want a plain do-not-hit-me style block.
That would depend largely on how the mechanics unfolded. If they ate the consequences, they would have no more to throw into the curse. If a concession as listed above is used, they don't get to dictate further action about a death curse if I have the fate token to spend. Also, I could shave some power off the block to add some duration.
Two reasons my character would want to stop a deathcurse before it starts, we want the guy alive... or as a shield for other team members that can't throw up a 16 shift block. They would only have to roll against normal spells.
If you're interrupting the spell after they've paid the cost, then they would still be dead, the spell would just fizzle. Death curses occur when the life force of a person is used to fuel a spell. In the books it seemed to be an all or nothing thing. Harry has never mentioned being able to use portions of his life energy. He prepares the whole shebang or nothing at all. (This is not to be confused with his soul when casting soulfire.)
It seems to me that in your situation, you have two choices:
1. You can stop the spell after they've cast it. The costs are paid, and they are dead whether the curse lands or not.
2. You stop them from casting their life energy at you to begin with. You effectively remove the option of them using their life energy as fuel for a spell. Mechanically, they can't inflict the consequences to cast the spell, so they don't die and there is no spell.
EDIT: wyvern's suggestion of a concession seems like an elegant work-around to the problem.
The concession is pretty good, this isn't bad either. Gonna run all this by my gm since it is his table.
Hm... Actually, I'd probably treat this as an aspect invocation rather than a ritual.
Why? Well, let's see here: if you take a target out, you get to narrate that take-out - and can easily narrate using your necromancy to hold them on the edge of life and death. In this situation, there's no chance of a death curse at all, because it's your take-out result and you're not killing.
The other option where a death curse can come up is if they decide to pre-emptively die and throw a death curse; mechanically, I'd treat this as a concession, and negotiate it (and the outcome of the curse) as such. And that's where you can offer up a fate point to invoke your aspect as a kemmlerite to change the available range of concessions.
Hrmm, this isn't bad either. Gonna talk to my GM.