Remove the prop?
Actually, there are a number of simple parts you can easily take with you from the motor that would make it impossible to START the motor. But, then you might ask what if the thieves showed up with their own oars? Or, worse yet, they simply tow YOUR dinghy with someone else's dinghy that they COULD start the motor on. Or, they could air lift the dinghy from a helicopter maybe ... or ...Actually, we take our motor off our dinghy when we're not using it. Kept on our stern rail (locked) or spends most of the time screwed to a nearby parking fence (locked).