Magnets are the source of the problem with CRTs.  eg the earth's magnet field or a speaker magnet.  These fields are fixed.   A demagnatizer or degausser is an alternating magnetic field that removes the magnetized areas of a CRT by using ac current to produce a constantly changing field.
Use an electric drill like a degausser if you do not have the proper equipment.  There was info on that in the forum previously.