I am personally not a fan of them, but did you try the magic eraser? Also, I have seen on TV shows that they use areosol hairspray to remove things from walls. You should test both on a small area that cannot be seen to make sure it doesn't damage the wall paper before you try it on larger areas. I also read online that you could try a new gum art eraser (look at an art supply or craft store) or if it looks like it is a grease based spot, sprinkle powder on the wall and allow it to absorb the grease then brush it off with a clean paint brush. I typed into google "How to remove spots from wallpaper" and about a million sites came up. I would try each idea until I found one that worked.