User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-4721065-20130112104250/@comment-4721065-20130113002815

Halo rings kill all sentient life forms. The Tyranids probably aren't even sentient (or the most basic ones aren't, anyway), as they're all controlled by the Hive Mind. Now, the Ring might kill the Hive Mind itself, but the rest of the horde will remain and multiply on its own, although it probably won't be able to move anywhere from there.

This is even worse with Orks, however, as they reproduce by spores, which are anything but sentient. As such, you use the Halo ring, then the new generation of Orks simply grows up and continues to multiply.

And you still haven't addressed what Forerunners are going to do with Chaos. Really, I think their only hope is if they're completely Warp-neutral or negative, but that doesn't have to be the case: humans didn't have psychics for 22 milennia BCE, yet they were Warp-vulnerable all along.