Entombment. Unless they have a nuclear reactor down there for energy, they can't grow food and recycle their water and live in a closed system. I'm sure the larger military bunkers have that but you're talking a trillion to build one, out of reach of private citizens, even the billionaires.
Permanently seal the entrances / exits and air shafts. Use it as a radioactive waste disposal.
Entombment. Unless they have a nuclear reactor down there for energy, they can't grow food and recycle their water and live in a closed system. I'm sure the larger military bunkers have that but you're talking a trillion to build one, out of reach of private citizens, even the billionaires.