Do I assume you have Planning Permission and Building Regulations to do this work on your home, because if not, you could be in trouble and the room be unsafe to use for human habitation, without certain safeguards! Do check it out before going any further with this room, with either an Architect or Builder or Local Authority. I could give you advice, but it might not be appropriate in your area or country!
Is the garage door needing to be blocked on the inside or outside?
It might be best to remove the garage door for several reasons:
From the outside they are generally drafty and uninsulated letting in unwanted elements (rain leading to mold), damaging the wall/room. Also they could let in insects & critters just trying to avoid the weather.
Generally outside walls of a building are load bearing and a garage door would be dependant on the frame of the wall and could eventually compromise the support with sagging.
From the inside, the hardware is just plain garage door visible hardware if it is still remaining. Some people do not mind the "quasi industrial" effect. For the best insulation and purpose it is optimal to remove it and create a finished off living space.
Best to totally remove, make sure you have an out side egress compliant to building codes. So in that space you may need to frame in a door or window then brick up or side the balance of area. Make sure you have all your building permits in order or it could void your home insurance.
Lock the door then put a piece of furniture in frount of it to hide it