Little blocks of wood about 50mmx20mm, 12mm thick with a lathe nail sticking through placed under their tyres, so when they drive off, they stick, the tyre will not deflate until they pull them out, which they will have to as the bump as the wheels rotate will make it too uncomfortable to drive. In this weather it will not be until they go onto clear roads so they will not know where it came from. I worked on a job in London in the early nineties where the site agent used to get this several times a month.
Don't to it to more than one unless you can handle the suspicion & accusations.