Mike. Immobilisers were suggested some years ago but the FAA did not like the idea at all!! Just something else to go wrong that could definately cause the engine to go quiet - just when you need it!!
Some years ago we put a hidden kill switch in a magnetos earth lead. Very effective but for reasons above, I doubt that would ever be approved!
It seems to me a chain wound around the prop and padlock takes a lot of beating for low cost and effective immobilisation!
Woomera