A solution of half vinegar, half water will clean the soot (and nicotine, other stainings) from your walls, and has the bonus value of freshening the air as well. Be sure to wash the ceilings too.
Can't help with cleaning, but have two suggestions for prevention in future. When anyone is smoking 1. light a candle nearby, the flames attract the smoke and makes it disappear, and 2. put/leave out a dish or cup of vinegar, it also attracts the smoke, you can tell because it turns black on top, when completely black, time for a new bowl.
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