What material blocks magnetic lines of force?
Nothing can block magnetic field lines. Magnetic field lines ALWAYS go around a closed loop, so says Gauss's law of magnetism. Some materials have special abilities, though, to warp or change the shape of this loop, and thus, in effect, gives the impression at the macroscopic level of hindering the magnetic field.
The only thing that will completely do it is a superconductor below its critical temperature exhibiting the Meisner effect.
ferromagnetic materials such as iron, nickle and others. These materials do not actually 'block' lines of force, but rather the lines of force go through them and therefore can be routed around something else. These are used in magnetic shielding to prevent magnetic fields from messing up something, like the CRT in a television set.
