Blocking stray RF with diamagnetic materials?
Ferrite is commonly used to absorb RF.  With the right formulation, it can appear to have the same impedance as free space, except that that RF waves are absorbed rather than continuing to propagate.  Ferrite is one of the two common means to make anachoic RF chambers, with the other being cones.  Often both ferrite and cones are used together to cover a broader frequency range.

Once you see the cost of ferrite though, you may want to rethink your strategy.