Post some boiler pictures :)
Its normal to leave one radiator without a thermostatic radiator valve, usually the one in the same room as the thermostat.
If you are...
Have you got a room thermostat? on the wall or free standing
Why do you want to turn it off?
Have you got one on the radiator in the same room as the thermostat?
I would ask the installer if it's a heat leak / bypass first before doing anything
If it is needs to be left open
Yorkshire YP3 Solder Ring Male Coupler 15mm x ½
At least he knew how to connect to Iron :D:D:D
Got to be a diy job. Look at it?
You can't heat part of it. Hit and cold water mix to give warm, it doesn't separate
What's your incoming flow?
Stop tap fully open, and the one in the road?
What boiler is it?
Separate names with a comma.