How old are these kids?
I'd tell 'em if you use the car without my permission, and put me at risk of
liabilty where we could lose our cars and home or kill someone, the cops
will be called for unauthorized use. Then I'd put the keys where I would
normally put them and that is that, unless you think a stranger is going to
break in your home and steal the keys and car, then I'd get a darn good safe
and put my checkbooks in there too because that also is a risk. The fuse
trick sounds interesting however, as does loosening the coil wire, or
putting in a dummy coil wire.

my .02 for what its worth.