The way I prevent gas theft from my car is I have a locking gas cap on the fuel filler hole which prevents the gas cap from being taken off without a key and I also added an additional lock to the gas door.
This prevents the gas thieves from easily accessing the gasoline in my car and I used to have a problem with people stealing my gasoline from my car even when at work etc and after adding the lock gas cap and locking gas filler door and no longer had my gasoline stolen.
The gas can still be stolen by drilling holes etc underneath the car but adding the locking gas cap and lock to the gas door helps prevent the gas theft because the thieves would rather take the easy way of getting the gas.
I also lock my vehicle in my locked garage overnight or when I'm not driving it as they also helps keep the car from being an easy target for thieves.
If you have no garage then add the locking gas cap and lock to the gas door and install cameras to help catch the gas thieves in action.