"Borrow" their car keys, then park the car in a really narrow alley or between two cars so that its impossible to open the doors , it helps if the car has a sunroof you can climb out of, or you can fold the back seat down, then climb out the boot/trunk and then put the seats back into position, closing and locking the boot behind you.
Takes ages for someone to figure out how to get in the car.
And it doesn't damage the car, at all.
Buy one of those metal whistlers and stick it up their exhaust.
http://www.the-joke-shop.com/showprod.asp?product=656
They'll get some right funny looks on the way home, they might even get pulled by the cops.
With regards to the shaving can, you can freeze a can, then use pliers to peel the metal off and stick the frozen foam into their car, 2 tins will fill a car.
Works better if you can immerse the can in Liquid Nitrogen first. (some Uni's have it in their labs).