Abstract: A method and a device to prevent unauthorized use of combustion engine driven vehicles, ships, equipment, as well as engines only, including the steps to remove at least one spark plug, to attach a device having an externally threaded part as replacement for the removed spark plug(s), said part having an embracing outside member, which by means of a locking mechanism can be brought into a freely rotatable state in relation to the threaded member, thus preventing removal of same, or, alternatively, bring said members into an interlocked relationship, thus enabling the device to be attached or removed from the engine.