How do I close these radiators?

I have a couple rooms on my second floor that I rarely use. The upstairs is always warmer than the first floor, and I'm only upstairs to sleep.

How can I close these? Is it as simple as turning the knob clockwise? Is there any risk of breaking the valve or causing a leak? This is a very old home. If necessary I'll wait until spring where I can drain the water before closing the valves.

Thanks