OB Lock - not a cable
I've used an OB lock that covers the two thumb bolts that secure the OB to the mounting board. I believe I bought it from West Marine. It is specifically made for this purpose but resembles a piece of channel iron that is permanently blocked at one end and a padlock secures the other. The device prevents anyone from unbolting the outboard from the mounting block. I've had it for years. Never lost a motor. If someone wanted to circumvent the lock, they would have use a hack saw to cut through it. It would take a while and would be pretty noisy and obvious. My concern with the cables is they are too easy to cut with bolt cutters.