Like they said, concrete the doors first.

What you dump down the air intakes afterwards is your business.

But, really -- who cares? Let them live out their pathetic lives in their sealed-off bunkers. As long as they never come out, who cares what they do in there? Let them live for decades in their coward-hole. Give them plenty of time to think about the decisions that led to this. Until, finally, their food runs out, their servants murder them, or they run into a medical issue that they don't have supplies to fix. And then the bunker becomes their tomb. And nobody will notice or care when they die, because to the rest of the world, they're already dead.

Bunkers will have ventilation shafts. Would be a shame if someone shit down it.