OT HVAC Experts - A/C vent in outside room

Go in attic pull duct off plenum and cap the duct and plenum, then close the vent. That way when you want air back just go hook that duct up. If it's not on a separate duct there should be a louvre.

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brew400

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Listen to this guy

Cool lawnmowers > Hot and tired lawnmowers

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Would blocking that vent lower my bill by pushing more air back into the house?

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Probably. All that air going into that room as it stands is unavailable to be return air for the AC unit. The upshot is that you're essentially throwing that air away after you pay to condition it. Usually, most of that air would be available as return air to the air handler, but instead you're having to bring in hot humid air to makeup what you're throwing into that room and hot humid air is way more expensive to condition than return air from inside the building envelope.

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If the door is left open, does that adversely effect how my AC cools the house?

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Probably. I'm assuming you mean a door between the storeroom and outdoors. The air that you are throwing into that room has to work its way outside the building through small natural cracks and such in the building envelope. If you throw the door to that storeroom wide open, you're giving the air a much easier path out of the building envelope. Since fluids are going to flow through the path of least resistance, you're likely going to find that when you open the door, the system is naturally going to end up wanting to supply more air to that space than when the door is closed. Unless the system is complex enough to compensate (unlikely for a home system), that air finding the easy path out through that door is likely going to be coming out of the air supply that usually would go to cool the rest of the house.

Now, that being said, the reason I started both replies with "probably" is that there's a lot going on in HVAC work and the expected results of events don't always happen if there's something out of the ordinary in the building or HVAC system.

EDIT: If you want to remove it, I'd disconnect at the trunk line and cap it there.

There is enough air in the house where he wouldn't have negative pressure or having a problem with his return , also I'm assuming this room is well insulated, just keep the door closed ...

If you don't want air in that room then just cap off the duct like mentioned before but imo it shouldn't affect anything going on in the house at all , of course if he closed it off there would be more air flow in the house but I don't think from the OP that he's having an issue keeping his house cool or that the unit is being over worked ...

If the room was built to be climate controlled it's just like any other room in the house , it's not like he's trying to cool off a Morgan shed ....