I hope this helps the way I'm going to try to explain it.
First: It isn't wise to eliminate any cold air returns because it would hamper your furnaces heat output. You must have at least the same amount of return air as supply air. The blower in your furnace actually sucks in the return to push out the supply. So if you have little or no return you will have no supply. Think of it as pulling a vacumn. If you put a straw in water and then cover the top, no water will come out. It's the same with your furnace. If you cover your cold air returns you won't get any heat out of your plenum. The burner will come on, then the blower, but the air will not move casing the furnace to go off on high limit. As the air slowly rises and cools your furnace will come back on and then shut down again. This will continue to short cycle with no heat getting out of your furnace. As far as cold air getting into the house they're all tied into the same return main, or they're supposed to be. I don't understand why they would be delivering cold air when the furnace is off. They're tied into the bottom of the furnace and cold air falls, not rises. As I said the blower pulls in the cold and releases the warm. You can take a tissue or something light and put it over the ducts to see exactly where the air is coming from and if need be call your contractor before diconnecting anything. Without proper return air you can burn out your heat exchanger. Which means another furnace.

I'd recommend you completely close off one return from the unheated part of the house. Just a minute, are there water pipes there that have to be kept unfrozen?
Any way I'd shut off one completely, you decide, then I'd seek maybe a shrink.
You're an idiot if you think you can non-heat a portion of a house without structuarlly damaging the other.
They have "tablets for that".

I probably shouldn't say this but what Josh wrote I think was a littte rude. That what these forums are for is to learn. If you want different temp. in different parts of the house that is your perogative. That is what we call zoning and it makes perfect sense. The less you heat the more fuel you save. As he did say though be careful of water lines. Befor shutting off a return though make sure you have enough to run your furnace properly.