Radiator

Turn off the valve at the other end.

Radiators (nearly always) have a valve at each end. One (which may be a thermostatic valve) is used to control the temperature, and has a knob which you can turn to adjust the flow through the valve. The other, called a lock shield valve, is only adjustable by removing the cap (which may just pull off or may be screwed on) and using a spanner on the flats of the valve spindle. The lock shield valve is used to balance the system so that no one radiator steals all the heat. If turning off the lock shield valve, count, and make a note of, the number of 1/8 turns necessary to fully close it. Then it can be re-opened to the same position and maintain the system's balance. Close the valve by turning the spindle clockwise looking down from above.

 

Will be helpful if you post a picture of the valve and plastic cap as a few different methods out there for turning off / adjusting rads

Some caps are purely cosmetic and need removing to gain access to valve underneath, they usually pull off or may need a flat screwdriver to ease off

Some caps clip onto valve spindle and turn valve on off, again pull off (but can be tight)

Other caps screw on to top of valve spindle, remove screw, cap lifts off

Thermostatic valves again are different

Post a piccie I suggest

 

Turning off either or both valves will stop the radiator heating.

 

I'm only diy myself but here goes (although I could be wrong)

1st pic looks like bottom section of a TRV - Thermostatic Rad Valve

Have you removed the top section yourself ? If so replace it and turn valve off - usually numbered so turn to zero

Although actual valve looks a bit corroded so may possibly be seized. A squirt of WD 40 wouldn't hurt

2nd pic - looks like the cap has been missing for some time as valve has been painted over. Simply turn off clockwise with spanner, grips, (will have 2 flat sections of spindle)

 

first pic does not look like a trv ,think its a wheel head with cap removed. Fiona ,can you do a frontal pic of that valve ? and can you actually turn that valve clockwise or anti clockwise ?

 

that is a very old wheel head valve ,and if it just turns and turns non stop ,then inside it is not engaging with the spindle .its had it I'm afraid.as the retaining screw head is "chewed " up ,it will be very difficult for you to remove the head. if all you are really needing to do is turn the rad off,so it will not heat up ,you can close down the left valve. which is the lockshield. take careful note of how many turns it takes to close this valve ,as it must be returned to the position it is in now ,when you need to bring the rad back into use. so for instance ,if it takes 2 and a half full revolutions clockwise of the spindle to fully close the valve ,then you would have to turn it exactly the same amount anti clockwise to bring the rad back into use ,and maintain the balance. with regard to the valve with the chewed up screw ,if you want to get it off ,the wheel head looks like plastic to me ,probably very brittle with age ,and would likely just crush off with a bit of persuasion .