Either of them. I think most cooldowns also work (Shield Wall, etc), it's not as specific as Ra-den was.
If it's for NM, know that if you have stacks when the boss dies, the stacks won't spawn any add. So yeah, if you're killing the boss soon and you have already taken the debuff once, you can also NOT use active mitigation to keep stacking until the boss dies.
If you mess up and accidentally get a stack and want to avoid having more adds spawn, then you can do as Senen suggested and let the rest of the stacks go through as well until he dies. You will take alot more damage doing this, so what I do if I accidentally run into that situation is I will use shield block immediately after an application of injected, that I way I can mitigate some of the incoming damage and it will fall off before the next application goes out. You can of course substitute barrier as well with the same effect.
<Uchirosan> Day 1 Prot Warrior
Resonate of Bleeding Hollow 14/14H 10 US 16