Had a similar thread I opened a year ago, I will see if I can find it.
I had a buddy that was getting the tank of his truck siphoned off. He eventually loaded a couple cans of gas in the bed of his truck and left them with a little something special.
A few day's passed and one of the neighbor kids was having their car towed away from a few blocks away where it died.
Don't remember what he used though.
From the research I did, if I remember this correctly, Paint Thinner burns much hotter than Gas. You fill it up with your cheapest stuff and top off that small gas can with 1-2 quarts of paint thinner it should pretty much do a good job of toasting his engine. If it dosn't kill it it should significantly shorten its lifespan.
The other one I thought of was styrofoam. The stuff dissolves in gas. It would stick to injectors and valves once it is burned off.
Perhaps a mixture of paint thinner, Diesil, styrofoam and 87 octane? Heat, knock, and crud build up inside the engine along with possibly coating his valves and Cats?