Magnetizing the dish will do nothing at all. Magnetism plays no role in the reflectivity of metal.
UDL
That is the right answer, and normally, it should end this thread.
True, Maxwell theory says something about electromagnetic waves having components of electric and magnetic vectors.
In microwave ovens, magnetrons, generating microwaves, contain strong neodymium magnets.
But, it is a fact that magnetization does not influence propagation of waves and reflectivity of metals.
Visible light is the same electromagnetic wave as radio waves, the only difference is the frequency and wavelength.
If the magnets could affect frequencies used by TV and/or satellite signals, therefore, they would have been used long time ago in microscopes, telescopes, and other optical devices.
The original post is somewhat confusing, because, instead of one, "clean" question, it sneaks in two different problems :
1. Magnetization as a criminal act
2. Does magnetization of a dish affect its performance