You can sand them down. But you need to use wet paper and water. This will allow the sandpaper to not clog and "polish" the paint down. To make it look like the rest of the wall, you need to play with the grit size until you find which one will match the paints sheen. You most likely will need to go over this area to get back the roller or brush marks also. Start with 240 grit and work with 400, 600 next. this is the black paper.. easy to find at an automotive supply store.