unfortunately there is no way to edit out room reverberations, only to add them.
I have just had to do the same thing that you are doing.
Usually the clothes will act as a sound deadener pretty effectively, bu my closet had the same problem because two walls (one of which had the doors on it) and the space above the clothes were bare and hard. The ceiling had that spray-on stuff that is not all that absorbent at all. Two routes you can go.
You can buy some sound-deadening foam (stuff that is really designed for the purpose, not matress foam) and put it on the walls.
Or go to an insulation supply store (you will likely have to find a construction supply company not a home improvement store for this) and get some unfaced rigid fiberglass ro mineral wool insulation (Owens Corning 703 or equivalent). This will likely run you seventy cents or so per square foot per inch of thickness and you will most likely have to buy a whole bale (a 2 foot high stack of 2 by 4 foot panels). Since the place I bought it from the cost went up linerly with thickness (two inch panels were exactly twice one inch panels) I just bought one inch panels since this meant I could make one, two, three, or four inch panels as I felt I needed. Use gloves and wear long sleeves when handling. cover the panels with an accoustically open material. Many folks like burlap. I used felt. Finding something that will stick the felt to the 703 was difficult My first thought was to use stickyback velcro...completely useless. My second choice was hot melt glue...satisfactory, but only because it won't be under a lot of stress. Use 1 inch to soak up highs, 3 inches to soak up highs and mids, and four or more inches, near corners, to kill bass rumble.