Hit the magnet repeatedly with a hammer or drop it to the ground in order to disrupt the order of the magnetic dipoles. Once displaced by the vibration, the magnetism will be lost.
Heat the magnet past its Curie point to remove inherent ferromagnetism. For most rocks and materials, this is above 570 degrees Celsius. Iron's Curie point is more than 770 degrees Celsius.
Use an alternating current to demagnetize a magnet by generating a current through a solenoid. By alternating the current, the magnet's dipoles will try to keep reversing and can't keep up; breaking the order of the dipoles and thus, the magnetism.
Go technical and use a magnet demagnetizer. While only useful for the tips of screwdrivers and other steel tools, it only takes seconds to magnetize or demagnetize an object. Insert the tip of the object into the small hole of the VTMD and rapidly shake for several seconds.