I've just been targeted by tyreextinguishers.com! They don't like my car. (If they ever find out I have four cars, one of which does 8mpg...!)
They take the cap off, put a lentil in and screw the cap back down. The air slowly escapes, tyre goes flat.
Their website very helpfully explains how to remove the cap in the first place, lefty loosey, righty tighty. Clearly they are as thick as planks.
And, yes, they really use a lentil! A lentil! Sorry, kat, but a lentil! I think that says everything you need to know.
Anyway, brilliant idea...removing the caps will mean they have to crouch down while the tyre deflates. Which they probably won't do. But that's not ideal for the health of your valves. So how about a dummy valve? It looks like an ordinary valve, protects the real valve like the cap, but confuses the heck out of people of low IQ.
(The regular anti-theft dust caps often use standard hex/allen keys, too easy to remove. I did think maybe smearing some dog poo...readily available at the side of the road...on the cap would be a quick solution.)
Lentil in bottom right of photo.