How to close off heater vent in dorm room? Help!?

If it is a floor register then take it out and wrap a plastic garbage bag around it. (Your furnace air should never be so hot that this would be dangerous.) You can then punch little holes to allow what ever heat you feel that you need in your room. If it is a ceiling register then use thumb tacks and plastic. Have a feeling that if this is a dorm room then these won't be your only thumb tack holes.