The answer is this: You will need to cover the item you don't want to heat with a conducting material, creating a so called Faraday's cage, which is connected to an earth potential equal to the potential of the metal casing of the microwave oven.
The reason for this is threefold: The covering material needs to be a conductor, because conductors have the property that free charges can move free without significant resistance inside it. Because of this, the electric field inside the conductor is zero, for elsewise charges would move due to the electric field. But they move themselves in such a position that they cancel out the fiel again. Now, because the electric field inside the conductor material is zero, is also has to be zero in cavities of the conductor. For if it would not be zero inside the conductor, one could move electrons through a loop partially through the cavity, and partially through the conductor. The path integral through this closed loop would then be non zero. In onther words, one could generate energy without costs, obviously in contradiction to the law of conservation of energy. Therefore, wrapping an item in conducting material creates a so called Faraday's cage, whereby the electric field through the wrapped item is zero.
Now this is what we want, because the microwave oven heats items with the use of electric field waves. So, shielding the item off from electric fields, also shields it from heating.
However, charges inside the conductor start moving because of the electric field waves. This can create a potential difference with the shielding material of the casing of the microwave oven. It is because of this difference that arcing could occur. That is why you will also need te connect the conductive wrapping to the earth potential of the metal casing.