How do you block cold air flow from an AC vent without blocking the vent?
They do make sheet magnets if your register is steel and not aluminum. Generally they have dampers in the registers for just such a reason. Just a word of caution, the heating and air conditioning system does require a certain amount of air to operate properly. If the system and duct work has all been sized properly then shutting off a small vent will probably not be a problem. But if the ducts are too small to begin with you could run into issues with the cooling coil freezing or liquid refrigerant getting back to the compressor. Or the furnace overheating. It is basically the same problem as a continuously dirty filter.