A magnet won't destroy a TV. It will put weird, swirly colors in the picture tube which will eventually correct itself on newer picture tube TVs. On older ones, the TV shop would be able to get that out with a coil made for correcting the color.
When I was a kid, I permanently damaged the bottom corner of our TV with a magnet. Nothing on the order of what happened in the show, but there was a permanent purple blotch on a 9 square inch area.
shareyeah, I know, but the coil I mentioned from the TV guy would fix that...
It's called a degaussing coil. It is about the size of a car steering wheel. It plugs into 120 VAC and it has a switch in the cord. You turn it on and move it around in a circular motion. You can "move" the purple stuff and other discolorations until they clear up. Some TVs had one built into the frame of the front cabinet (as in a cord that went around the edge of the tube) and they'd keep that stuff out of your tube. Older, or cheaper, TVs didn't have it. Yes, I used to work on cheap TVs at an Emerson Radio Corp. location. :)
Anyway, when that was done with a magnet, you usually could get it out unless it was there for a very long time. But tubes would go bad and they could get screwed up colors...
anyway, that's all in the past now. :)