Calling a private method of other running app

I want to call a private method of com.android.bluetooth.BluetoothOppService StopListener() to prevent the system service BtOppService from accepting incoming bluetooth OPP request. I know how to call a method by instantiating a class myself and invoking methods using the instance, but I want to call the method using existing instance of a loaded process. This could be one great answer to my question, which asks the way to stop a system service.

Thank you in advance.