A protective device (26, 28) is proposed that contains a protective generator (26) and an inductor (28) to generate an electromagnetic protective field (29) that is suitable for compromising the function of the spying device (SM), wherein the protective device (26, 28) generates an electromagnetic protective field (29) with a protective signal that simulates the type of signal that occurs when magnetic stripe cards are read. Through the protective field a special protective signal is induced in the potentially present skimming module to imitate the reading of the magnetic stripe card. The skimming card reader reads a pseudo magnetic stripe card, or is at least confused when spying on the genuine magnetic stripe card. In particular, the protective signal can contain pseudo data that are mixed with the actual card data at demodulation or suppress said data completely (capture effect) so that the third party ultimately obtains unusable data.