Screen burn is caused by the burning off of the phosporous coating inside the front of the tube.
One it's burned off, you can't fix it. The only thing you can do is swap a new tube in, or buy a new monitor.
You can *hide* it by putting a dark tinted piece of acrylic (or car window tint on glass) and laying this between the monitor and glass/plastic bezel. This way you can't see the etching unless you have a very bright single colored image on the screen.
Unfortunately, when you do this, you will need to turn up the brightness control to compensate for the darker screen, and this will end up burning in the screen faster.