This will delete unit's and vehicles but not objects placed as triggers don't usually check for objects unless grouped to them.
TRIG
Set the size of the trigger to cover the area you require
Anyone
Present
ON ACT
{{deleteVehicle _x} forEach crew _x + [_x];} foreach thislist]
This is a bit more complex but quite usefull.
TRIG
size 0
name Trig1
Cond
player distance Trig1 > 100
ON Act
what = nearestobjects [Trig1,[], 33]; { {deleteVehicle _x} forEach crew _x + [_x]; } forEach what;
[Trig1,[], 33]; Trig1 = location in this case named trigger, [] = in this case anything it can find, 33 = radius