What can block a magnetic field?

Steel is good. There is a special metal called mu-metal

Mu-metal is a nickel–iron soft ferromagnetic alloy with very high permeability, which is used for shielding sensitive electronic equipment against static or low-frequency magnetic fields. It has several compositions. One such composition is approximately 77% nickel, 16% iron, 5% copper, and 2% chromium or molybdenum. More recently, mu-metal is considered to be ASTM A753 Alloy 4 and is composed of approximately 80% nickel, 5% molybdenum, small amounts of various other elements such as silicon, and the remaining 12 to 15% iron.

A big chunk of something that isn't magnetic.

A thick enough layer of ferrous metal, a single area piece blocking between _both_ poles of the magnet and whatever you are trying to block it from. It could be a single thick piece or several thinner layers.

As long as it is thick enough in proportion to the strength of the magnet, the magnetic flux will extend to / from the magnet and along the metal, not passing beyond it.

A layer or sheet of superconductor can also block it, but that's usually somewhat impractical.