When World War II began, Germany immediately banned listening to foreign broadcasts. This was difficult to enforce, although there were a fair number of cases in which people turned in neighbors whose radios they overheard were tuned to enemy stations. Another way of dealing with the situation was to insist that radio receivers have a tag on a knob warning people against listing to the wrong stations.
Nazi block wardens, the lowest echelon of the party’s apparatus, were expected to check on the presence of the tag when visiting. This was not popular.