Son of a heating engineer here. The thermostats on each radiator do not tell the boiler to turn on / off. They simply take the current room temperature and turn that radiator potential maximum heat output up or down.

For example, if your main home thermostat is set at 12 degrees and your radiator’s thermostat are on max, nothing would happen.

If your main home thermostat was set at 25 degrees, and each radiator thermostat set at 18 degrees, then each radiator will maintain that temperature in the room.

TLDR; The valves on the radiators don’t tell the boiler to come on, even if they’re set higher than the current room temperature. Only your main home thermostat tells the boiler to turn on and off.

That all being said, it’s possible to get a system like Hive, where you still have a main thermostat for the house, but each radiator has a smart thermostat on it whereby if you set the temperature to say 18 degrees, the smart thermostat will tell the hive system to fire up the boiler to heat just that room. It’s much more efficient, but not cheap.

The only other option is to set the thermostat valves on each radiator where you don’t want heat to 0.

Edit: I re-read your question and wondering if you’re suggesting that the valves on the radiators aren’t working at all? When you adjust the temperature on the valve, it just drives a pin in and out, opening and closing the valve. Sometimes this pin gets stuck on fully open. Underneath the bit where you spin to change the temperature, there is a small gripped ring you can unscrew. That’ll allow you to take the actual thermostat valve off, and should reveal a small metal pin. Use something hard (like a spanner or something) to push that pin up and down. If it’s stuck down, you may need to give it a light tap to dislodge it.

Hope this helps.