@Giallustio implemented it by two way, the simpler, for me, is explain here:

//if you want a safe area
if (_position distance getMarkerPos "YOUR_MARKER_AREA" < 500) exitWith {};

Note : you can tweak radius of desactivation by changing 500 meters by more or less. Make sure your marker radius is large enough to reach out and overlap the epicenter of the town you want to black out.