How can I stop heat loss due to an uninsulated porch?
You are correct that having a hot radiator on what's effectively a cold space - the porch - is very inefficient and, presuming nobody's on the porch most of the time, wastes a lot of energy.

And you are correct that the hot water going into that radiator is being cooled in the process.

However, it is unlikely that the water is going to any other radiators, but rather it's going back to the boiler to be reheated.  Nevertheless, you are right that stopping water flow to this radiator will save you on your heating bills.

But - if you stop the flow of water through the radiator, no amount of insulation will keep the pipe from freezing, because with no place for the cold water to go, the hot water in the rest of the system WILL NEVER reach the pipe you insulated.  It's just like turning on the hot water in a cold bathroom - the water runs cold for a while even though the hot water is available elsewhere in the house.

On a cold enough day, the stopped pipe with water in it will freeze, no matter how hot the rest of the system.

The best solution to this is to remove the radiator completely.  To accomplish this, you need to trace both the input and output pipes for the radiator in question back to an interior space that doesn't ever drop below freezing.  The entire system will then need to be drained, and the input and output pipes cut off and capped so that the useless radiator can be removed.

EDIT:

Since you're looking for a short-term, this winter solution, here's what you can do for now to minimize the waste.


DO NOT TURN OFF THE RADIATOR
DO turn down the radiator to as low as you can get it without shutting it off.  You don't want it to have a chance of freezing.
Wrap the radiator in blankets to keep in the heat it does generate and conserve energy as much as possible.