This frustrates all of us and very sympathetic to your loss
There were a number of posts on Hitch Locks and Wheel Chocks a few weeks ago to protect against someone hitching and towing away but this would not prevent someone from backing a flatbed up and driving off with the trailer on board.
I have a set of rotochok
locked on the wheels
and on the Hitch, coupler & reciever I have Products - Towing Security - 3794DAT - Master Lock®
others have discussed using a handheld tug to point the hitch away from the street/driveway
None would defeat a flatbed tow truck however...