Rub a dry cleaning sponge on the walls, starting at the top and working your way down. Do not wet the sponge; simply rub it on the walls. Wear rubber gloves as you work.
Cut off a layer of the dry cleaning sponge as it becomes clogged with soot. Use a sharp razor to cut the top layer off and to reveal a fresh layer. Continue working.
Wash the walls with a household cleaner or cleaner specially designed to remove smoke odors. Fill a bucket with the cleaner after mixing it with water according to the manufacturer's instructions. Wash 1-foot square sections of the wall at a time using a regular sponge.
Rinse the wall with plain water and a clean sponge to remove any cleaner residue. Let it air-dry thoroughly.