Make shift kill switch?
I'd just take the coil wire off.
Seems pretty inconvenient to have pop the hood every time you park the car and then again when you come back. Works fine for a car that is only a toy, but if you want to use it as a daily and run errands or travel I could see that being a pain.
To the OP, I would bet that introducing a vacuum leak to keep the car from starting would only be marginally successful. I've driven my car after a backfire popped a large vacuum cap off and while it didn't drive well, if I gave it enough throttle it would run. It would probably slow them down, and might even cause them to give up, but not sure. Either way, an electrical disconnect is easier to hide, but even then if there is power under the hood a couple of feet of wire and quarter is enough to get it running assuming there isn't a fuel disconnect as well.
The reality is, all you can do is increase the chance that the car will be there when you come back. Nothing is perfect.
