There are a lot of ways to do that;
here's one of them:
Create a trigger, covering the area, where your civie's are
supposed to be.
Activated by: Civilian
Condition: this
Onactivation: civilist = count (thislist)
Name this trigger by using it's name field (let's say: triggerA)
Now create another trigger:
condition: count (list triggerA) < civlist
This trigger should become active, once one of the civi guys
inside the trigger area has been removed from that earth
~S~ CD