Make a solution you are comfortable using. Mix a commercial paste remover with water according to manufacturer's directions. Another choice is 2 to 3 tbs. of liquid dishwashing soap, or a heaping tbs. of baking soda, or 4 tbs. of fabric softener, mixed with a gallon of warm water.
Use a sponge to apply the mix you choose to the paste left on the wall, working in a small area. Let the solution soak into the paste and gently scrape it away with a drywall scraper.
Use a pole sander with 100-grit sandpaper, if there is any paste left on the wall.
Sand with an electric sander, if the pole sander did not remove it completely.