Staggering number of replies with 'license' in place of 'licence'
Even those posting links with the word 'licence' in the link use 'license'
licence = noun
license = verb
(I am the spelling police today; clearly the government need to spend more on education)
Anyway, speaking as someone who has more than a passing knowledge of this, it's highly unlikely those nice vans with the rotating roof aerials can detect your using a television.
In the good old days, when a television used a cathode ray tube for a screen, rather than the LED/LCD/Plasma screen 99.9% of us have now, there were certain things required to make said screen work.
One of these was a line-output transformer (LOPT) The LOPT required a voltage of around 25,000 volts (could be as high as 29,000 volts) for a colour screen, or around 8,000 volts for monochrome (Black & White) These transformers would in turn generate their own radio frequency (RF) signals ('cos that's what high-voltage stuff does) and the detector vans' aerials, could supposedly pick up these RF signals and determine from where they came, by using triangulation, therefore even in a block of 100 flats, no problem to say it was dear old Ethel in Flat 89 on the 22nd floor. There was also - supposedly - a way of detecting the signals emitted from the Local Oscillator Crystal (which operated at ~4.33Mhz) and would therefore confirm that you had a colour telly, even though you'd bought a B&W licence.
Having gone off at a complete tangent, to answer your original question, just speak to them in French until they go away (that's what I do to the JWs)