One of these devices doesn't work, so one radiator is at full blast
the whole time

For a quick and dirty fix, just crank down the "lockshield" valve at the other
end as much as it takes. It will probably have a plastic cap over it, push-on
or secured by a screw, under that will be a valve stem with flats on it that
should turn with a spanner / mole, or pliers if you are lucky. It's just a tap.