Use a Faraday cage to block electromagnetic transmissions from a device

A Faraday cage or Faraday shield is an enclosure used to block some electromagnetic fields. A Faraday shield may be formed by a continuous covering of conductive material, or in the case of a Faraday cage, by a mesh of such materials. (Source: Wikipedia)

Faraday bags are portable containers fabricated with metallic materials that are used to contain devices in order to protect them from electromagnetic transmissions for a wide range of applications, from enhancing digital privacy of cell telephones to protecting credit cards from RFID skimming. (Source: Wikipedia)

There are many products out there that claim to be a Faraday cage/bag, so it's important to verify that the product adheres to the IEEE Standard Method for Measuring the Effectiveness of Electromagnetic Shielding Enclosures. Make sure that the product is certified to shielding effectiveness standards MIL STD 188-125 and IEEE 299-2006.