I have a radiator I'd like to remove while decorating. At one end it
has a thermostatic valve, at the other end a plain valve (if that's
what you call it?). The plain one has a plastic cap that removes
easily, revealing this -

http://i.imgur.com/bH2il3H.jpg

I.e. there is a brass thing with a hexagonal head that I think I'm to
turn clockwise with a spanner until it won't move, shutting off the
valve.

If that's right so far, what if anything is the purpose of the screw
in the middle of the brass thing? It's a bit blurry in the picture,
but it seems to be the head of a screw that goes into the axis of the
brass thing.

Should I attempt to loosen or tighten the screw, or remove it, or just
pretend it's not there?

Thanks