Is there any substance that can cut off the magnetic field lines?

Magnetic field lines are artificial imaginary lines.

It doesn't actually exist.

Under external conditions, an object can produce magnetism as long as it can make the electrons in the atom change from discontinuous to ordered. First of all, you should understand magnetism, which is a phenomenon of mutual attraction. For example, a magnet attracts an iron block, which is the process in which electrons rotate around the core inside the iron block when the iron block enters the action range of the magnet. This new sequence has the opposite effect to the magnet. At this time, the iron block has also become a magnet, and the place where the magnet and iron contact quickly has two poles respectively. The polarity of a magnet is pre-existing, as is the case with all magnetic things, including the earth, in which iron accounts for a considerable part (any star), and the core temperature is extremely high, and then the movement of electrons in atoms becomes orderly at high temperature, thus generating the geomagnetic field.