How are magnets destroyed?

Ferromagnetism can be impaired by creating an opposing magnetic field. A wire coil placed around a magnet creates an electromagnetic field. If this is aligned in the opposite direction, it could eventually remove the intrinisic field of the permanent magnet.

In many cases, simply heating a magnet would likewise remove the magnetic field.

Use a degaussing coil (driven by AC, not DC). Gradually reduce the field strength either by reducing the AC current or slowly separating the coil and the object.