You can use magnets to destroy monitors and tv sets -- however:
I once "ruined" a monitor, not knowing that my colleague who used it was a former tv repairman.
He looked at the, now colourful, screen, then pressed Degauss a couple of times. It didn't help - but he did not lose his cool for a second.
Instead, he asked me for the magnet.Putting it in the middle of the monitor, he started to wave it around in the front of the monitor in a spiral pattern, very small at first, slowly increasing in size until he was "windmilling" his arm. Two more repetitions - and
the colour seams were gone...
Later, he told me the secret:
TV repairmen use a mains powered spool to create a strong magnetic field to clean up a picture. The procedure is similar.