Rent a wallpaper steamer and soften the paste that way. The intense heat of the steam should soften the paste enough so that it can either be peeled off or scraped off.
Scrape off any paste that has hardened with a razor blade and won't come off with steam. If you have a large section of dried paste that you are trying to remove you may need to chisel it off with a 1-inch nail and a hammer. However, keep in mind that this may cause damage to your wall and then you will have to repair those chips.
Sand the remaining pieces that you could not get off the wall with 150 grit sandpaper and a palm sander.
Texture your walls to hide all remaining paste that will not come off with a high quality wall texture. Make sure that all the wallpaper paste is at least sanded down into a smooth form.
Mix 1 gallon of hot water and 1/4 cup of tri-sodium phosphate (TSP) in a large cleaning bucket. Apply to wall with a large sponge and let sit for 5 minutes. Scrape paste off with a razor blade.
Add 2 cups of hot water, baking Soda, and fabric softener to a spray bottle. Shake it well until all ingredients are mixed well together. Spray onto the wall and remove paste with a sponge. If the paste is too hard to come off with a sponge, scrape it off with a razor blade.
Mix 1 gallon of hot water and a squirt of Dawn dish soap into a cleaning bucket. Squirt lemon juice directly onto a sponge or lint-free cleaning cloth. Soak this in soapy water, then wipe off the paste.