Please suggest a Hitch Bike Rack Theft Protection Solution
As suggested in the comments, removing the rack is by far the most secure option.
The hitch rack pin can be replaced with a locking pin. Alternately if the end of the hitch is accessible and sticks out past the end of the hitch socket, a padlock can be installed.
A chain from the rack to a fitting on the car that prevents it from being removed is possible. This may require having a suitible securing point mounted on the car.
All these help protect the rack, but a determined thief will defeat them all. My hitch is bolted to the car with just 6 bolts. 10 minutes with the right size socket and/or spanner will easily have the whole thing off the car and on its way to a new home.