Would-be saboteurs will have to come up with a different way of preventing people from driving off in their cars. Most foreign matter has no effect on modern cars, since the fuel has to pass through several filters on its ways from the tank to the engine. Sugar, in particular, would be useless since it would come nowhere near the filters, let alone pass through them, because it is not soluble in petrol. It would simply sink to the bottom of the tank. The only way of stopping an engine with sugar would be by adding so much that it completely covers the tank outlet, preventing any petrol from reaching the engine. Sand would do that job just as well.
The old movie trick of sticking a banana into the exhaust pipe to stop a car is also a myth. In no time at all the pressure created by the exhaust fumes would forcibly expel the obstruction.