This sounds strange, I have never seen a magnet cast shadows. Is this a magnet from a library, if so which one? Or are you applying it to a body part or prop from the Object menu? Or is it loading in a pz3? You are not giving us much information to go on here.

If the magnet is from a props pallet you could try opening it's pp2 in a text editor and searching for any lines starting with "castsShadow" the value for these lines should be 0, if it's 1 change it to 0.

As a workaround you could try hiding the magnet with the Visible check box in its properties dialogue, before you render. Or set its materials properties to transparent in The Materials Editor.