Create a trigger over the area. Group it to the player unit and you will find new options in the Activation box. Select group leader (ie player) not present.
On Activation: runAway=true
Now, in your dialogue, just check to see if the variable runAway is true: if it is, the dialogue ends.
So in your dialogue script you'll have a line
runAway:exit
If you are doing it with triggers then the condition line of the trigger will look somehting like this
Condition: (Whatever you have already) and not runAway