Harold Kestenholz
If it is a hot water radiator, then there is a water inlet and a water outlet. They both have to be turned off because water will come from either side. Turn the water feed to the boiler off so new water does not enter the system. You did not say whether there are other radiators or pipes higher than that radiator. If there is no other valve to shut off both supply and return to the radiator, then all the pipes and radiators above the radiator in the whole system will have to be drained to the level of THAT radiator.
