Brush off any loose soot on the wall with a clean towel.
Scrub smoke on the walls, using a dry, chemically treated soot sponge. Work in long, smooth strokes from left to right. Clean from the top of the stain down to the bottom. Switch to the clean side of the soot sponge when you dirty the first surface. Don't continue scrubbing with a dirty sponge, as soot will simply transfer to the wall. Once both sides are dirty, expose a new side by shaving off the thin top layer with a razorblade.
Spray a degreasing wall cleaner or all-purpose cleaner on the walls and scrub with a clean towel. As you dirty one surface of the towel, move to a clean section, spray, then continue cleaning.
Wipe the walls dry.