The best solution might depend on how you want to define the zone.
If you know the position of the zone in the Camera frame before hand, you can filter the detection inside the C++ code before sending them to UE. All you need to do is checking if the detection is inside that zone or not.
Or you can do it inside UE, in the level blueprint where the Actors are instantiated. If your zone is defined with a plane in the scene for example, this solution might be better.
There is also a parameter you can set (ObjectDetectionParameters Class Reference | API Reference | Stereolabs) to clamp the detection range.