Good conductor attenuate the EM wave, don't necessary block the EM wave. It depends on the thickness and the frequency of the EM wave. For good conductor:

[tex]δ=\alpha+j\beta\;\hbox { where }\; \alpha=\beta=\sqrt{\pi f \mu σ}[/tex]

Attenuation is

[tex]e^{-\alpha z}=\frac {V_{out}}{V_{in}}[/tex]

As you see, it is frequency and conductance dependent. Set the thickness z in the equation to get the desired attenuation( Vout/Vin).

http://en.wikipedia.org/wiki/Skin_effect