If you don't own a TV you will have to prove that fact to them or they will prosecute. If you own a TV and it is just not being used then you will have to find some way to prove that too.
Its a frightener. There is absoultely nothing they can do. Howver I sould suggest if you have a TV you don't watch it as I suspect they will send the Detector Vans around just to make sure.