Is your wallpaper, paper or vinyl? Try using a smoke sponge FIRST. If it is vinyl use a de-greaser after the sponge. Mr. Clean makes one. Make sure the label says it is a degreaser. Soot is an oily substance, plain water or cleanser without a degreaser can make the stain worse. I was a Project Manager for a Disaster Restoration company, so this question is right up my alley. You can find a smoke sponge at most cleaning supply stores, or some home centers. If you can't find one, do a google search as there are many sources.