Hang a painting, photo or mirror over the black Sharpie marks. If you're dealing with more than one wall, stagger the hanging objects around the room to form patterns or geometric shapes.
Place a piece of furniture in front of the mark or marks. For example, if a large portion of a wall has black Sharpie scribbles, place a bookcase or entertainment center in front of the wall to block the marks.
Cover the black Sharpie on the walls with paper or fabric wallpaper, paneling or paints. If you decide to paint over the marks, use darker colors and consider blotting the surface of the wall with the paint rather than using a roller or brush as this may blend the marks into the design you create.