Strong cable lock though a closed loop on the car the cars tie down loop eye around the bikes threaded to limit the range of twisting and lever motions meaning the racks locking mechanisms cannot be push further than they can cope with.
How to lock a bike to a car bike rack.
Attach a u lock or chain around the rear wheel the frame and the bike rack as above.
Not only does it make thieft more difficult but it a visual deterrent to bike thieves in the first place.
Use a u lock to lock the back tire and frame around the pole you are locking it to then lace a cable lock through the front tire onto the u lock and then lock the u lock.
You now have a bike to car lock.
This should help protect your bike your car and your carrier.
You can have an externally accessible tow loop retrofitted to the car and then you could lock the bike with a cable through that loop.
Google your vehicle make and model and add the words tow hooks or tow points for ideas or talk to a qualified auto mechanic dealer.
While you can be sure the bikes are safely attached to the rack thieves are equally as sure that bike racks can be removed too.
Then push one end of the cable through the front wheel pass one loop through the other loop and secure the first loop to the main lock.
Many bike racks include a locking mechanism but an additional lock is always a good idea.
A hitch rack provides a good lever that can be effectively used to snap the rack or the hitch off to take the bikes and if the bikes are damaged in the process the thief just walks away leaving a mess.
The insulation is totally optional it just depends on how worried you are about scratches.
So make sure your carrier is also secured to your vehicle.
Check that what you are locking your bike to is strong and sturdy.
From here just thread the padded chain through the trunk loop the bike carrier and your bike and padlock the ends of the chain together.
The several lock systems that i mentioned you should get are not just for your bikes but for your bike racks too.