If your receiver-mounted bike carrier does not lock to your car's square receiver, then Thule's Snug-Tite Receiver Lock might just give you an option for locking your carrier to your vehicle.

Suitable for use on 1.25" (32mm) and 2" (51mm) square receiver hitches, the hitch pin threads into the thread provided in your Thule hitch so as to minimise movement and then a keyed lock attaches to the other side preventing the hitch pin from being removed. It is suitable for use on 2.5" (64mm) hitches but we do not carry the adapter to reduce the hitch down to 2" (52mm) to suit your bike carrier.

This product may be suitable for other hitch-mounted bike carriers due to the standard size of hitch pins but the thread may not be available on all carriers.