Locking a vehicle bicycle rack
Don't rely on the racks locks alone to keep you bikes safe. They willat very best, slow the thief down (shows the point).  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. 

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.   

For any length of time, take the bikes off the rack and store safely inside, or out of site locked to a secure rack. Cheap bikes also help.