The cold and hot supplies to the taps have nothing to do with the radiators. The central heating (radiator) system in a communal system will normally have water in it at all times.
There may be valves to turn off the supplies to your flat, but they will be on the heating pipes, not on the hot and cold supplies. If you can't find these valves, or there aren't any, or they are stuck, I'd suggest the job can't be done in the way you envisage it. Try asking your neighbours and / or the management company if there are valves, and if so, where they are.
If you do undertake the work yourself:
1. You will have to turn the water off to the heating circuit in your flat (see above for valves).
2. You will have to find a drain off point / valve and drain the water from the heating system. Turn off all the other radiators at both ends before draining. Make a note of the number of quarter turns necessary to close each valve, so that they can be opened the same amount when you refill.
3. Undo the valve connections on the radiator with the faulty valve very slowly. There will be some water comes out, that which is left in the pipes. However. be ready to tighten the connections again if water doesn't stop coming out.
I think you would be well advised to get a heating engineer in who can freeze the pipes going to the affected radiator and can thus change the valve(s) without all the foregoing fuss. I personally wouldn't trust a freezer spray on a communal system - the consequences of failure are too expensive. An electric freezer should be OK.
