The majority of designer radiators on the modern market incorporate thermostatic radiator valves (TRVs).
For radiators of this type, you will first need to turn the valve down to zero, or all the way clockwise until it won’t turn anymore for valves that are not numbered. This will stop the radiator from heating up the room when the central heating system is switched on.
To prevent water from entering the radiator, the lockshield valve has to be switched off to isolate the radiator. Remove the cap from the top of the other side of the TRV to do this. Then, use pliers to turn the exposed metal spindle all the way clockwise and switch the radiator off.
To gain a deeper insight, explore our blog, How to turn a radiator off.