I have a radiator in the middle room of my house which heats up more
than the radiators in the front & back room?
I tried turning the valve both ways but it doesn't seem to affect anything?
Would replacing the shut off valve solve to problem or capping it?
I would say you just cant cap it cause you have a hot water loop and by removing and capping you will most likely really mess up the rest of the system. You MAY be able to pull the top of that valve off and replace the washer inside. IT WILL NOT BE EASY, THAT HAS BEEN THERE A LONG TIME and it LIKEs it there.
Much as you dont want to in this case I think you really need a plumber, depending how the system is run, you may be able to get under it and re plumb it to bypass it, but you cannot just do that without really knowing how the system was done.