It depends on how its burned in. Burn-in is nothing but unever phosphor wear. Therefore, you have to even out the wear to the rest of it.
If its burn in from 2:35:1 movies, then you have to burn in the bars at the bottom. Can be a bit tough to do such a thing, but a ton of fiddling in photoshop with tv-out, could do the trick.
If its from a station, logo though youre screwed.
Personally, my tv has burn in on the sides, because the idiots that owned it before me couldnt get the aspect ratio control right on the STB. Its been a bitch to find something to line up to those bars.
And letting a flat grey image run on the CRT will do ABSOLUTELY NOTHING. Neither will rapidly moving images. Technically you want to run an exact inverse image, but if its from black bars, the white IS the inverse image.