1. Disabling cook-off fire for individual vehicles
Cook-off fire can be disabled for a specific vehicle (does not affect ammo cook-off):
_vehicle setVariable ["ace_cookoff_enable", false, true];
Mission settings will always apply however, so you can’t enable cook-off on a vehicle if the mission settings have cook-off for vehicles disabled.
2. Disabling ammunition cook-off for individual vehicles and boxes
Ammunition cook-off can be disabled for a specific vehicle or box (does not affect cook-off fire):
_vehicleOrBox setVariable ["ace_cookoff_enableAmmoCookoff", false, true];
Mission settings will always apply however, so you can’t enable ammunition cook-off on a vehicle or box if the mission settings have ammunition cook-off disabled.