I have a yakima quickback that I've used for years. It's got a flexible fabric wrapped steel cable (similar to a laptop lock) that attaches to the rack and feeds through the back of the deck lid on a sedan trunk that makes it so you can can't pull the rack off the car without opening the trunk. It's not going to stop a pair of bolt cutters, but it's enough of a deterrent. There are also a few places that you can wrap a good cable lock to secure the bike to the rack for security on pitstops.