TV detector vans
TV detector vans were entirely possible in theory. TV sets (at least, all but the first pre-war, single-channel monsters) use superhet circuits for the radio reception side of things, and there is always some unwanted leakage of the local oscillator. And the L.O. frequency is by design a constant difference from the received frequency, so it's even possible to infer what channel might be being watched.
Now, if the detector van was using a superhet to pick up the local oscillator of your TV, then it would have been possible to build a TV detector van detector; which could cut the power to your TV when it detected a TV detector van in the vicinity, and restore it once it had driven off.
Although it would be a good idea to build your TV detector van detector using a straight TRF circuit; otherwise, it would be possible to fit the TV detector van with a TV detector van detector detector, possibly requiring you to spend more than the licence fee on the necessary low-noise components to build a TV detector van detector detector detector .....
Of course, whether any of this was actually necessary in practice, given that not everyone has the relevant knowledge of RF engineering. It's probably enough just to stick a scary-looking aerial on top of any black van, drive it around the streets for half an hour and watch the queues building up in the local Post Office ..... and the fact that it's all entirely viable in theory is good enough for the smarter ones.