I have an old rusty radiator that is leaking badly. I only want to temporarily remove it until it can be replaced but one of the nuts that attaches the valve to the radiator simply won't budge and so my only option is to completely remove the radiator with the valve attached (they don't work anyway). This leaves a copper screw fitting exposed. Is there anything I can fit to this (I've tried a 15mm stop end and it doesn't fit) or do I need to cut the pipe beneath this fitting and then cap. I think the system is a open-vented one (back boiler).