help! how do i deal with this?

I already tried to seal all the entrances to the cave and light stacks of wood on fire, but I underestimated how fast they can dig trough the layers of walls i built, and they made a hug gap to a roofless "courtyard' near the bottom left of pic related. now i don't think i can reseal them in without aggroing them. I already lost half of my colonists to this menace, which I ignored too long.