I've closed one of my domes because I'm low on water. However, colonists keep going in there and suffocating. What's going on? Why are my colonists going into closed domes and dying?

Edit: Maybe they're not going in there; maybe they're just not leaving. Shouldn't everyone evacuate a closed Dome, if there's somewhere else to go?

Closed domes prevent entrance and exit. You can instead set the filter to disallow anyone of any age from being in the dome and they should all move out. Once the dome is empty, you can then quarantine it.