how would you prevent theft in a simple way other than a kill switch?

if you just want something quick and simple, removing the starter relay or engine computer's fuse/relay is generally enough. Just make sure to remove the ignition side fuse for pcm and not the keep alive memory fuse. Cant imagine you would cause any damage other then maybe widening the terminals in the fuse box more then normal

Having to open the bonnet or hood to remove / refit said fuse or relay is so* obvious and time consuming same as grovelling under the dash for a fuse or relay there. May just as well remove a coil lead or the dizzy rotor arm which was done on older cars by some.

I put a kill switch on a Volvo 240 in the row of standard switches, in fact it was also a spare standard switch (from a scrappie) in plain sight - so easy to see but hard to work out the function.

Of course, a denver boot is a possible solution (wheelclamp), but a louisiana spanner is a solution to that...