You already got several suggestions, the professional one would be using
the degaussing coil.

As recommended by some degaussing tools; start far away, two or three
foot from the screen, turn power on to the coil, do wide circles in
front to the screen, two or three circles, then advance few inches
toward the screen, repeat two or three circles, keep going until you get
10-12 inches from screen, do 4-6 circles then go back repeating the same
sequence until you reach two or three foot away, =THEN= turn off the
magnetic monster.

If you are unlucky to turn off the magnet close to the TV, the problem
will intensify and you need to repeat all over again, probably two or
three times.

You can't turn it off close to the screen, because the strong magnetic
field interrupted will "mark" the metallic elements at the screen,
increasing the problem.  This happens because the degaussing tool uses
alternate current (120Vac) to create also an alternate magnetic field
(60Hz), it reverses the magnetic fields north and south sixty times a
second, when close to the TV that field is very strong to turn all
images very green or any other color, but as they are reversing, one is
canceling another, and this is why you need to first approach the screen
slowly, and then also get back from the screen slowly, they will "wipe
out" any previous magnetic marked field, they are stronger.  If while
close to the TV power turns off, or if the power cable is not well
connected at the wall power outlet, creating a "turn off" condition, the
last magnetic field will be "marked" at the screen... be careful.

Wagner