There are some pastes that can't be removed, but most are water soluable. I use a compression sprayer filled with a solution of water and a non-sudsing detergent like TSP or and equivilent. Saturate a wall with this stuff and take 5, then saturate again, then take another 5(I'm starting to feel like Dave Brubeck), then do it again, then 5 again and then test it with a putty knife to see if it is soft enough to be scraped off. I pull a 5 or 6 inch taping knife over the surface to get most of the adhesive off, scraping the knife into a drywallers mudpan. Draw the knive over the wall the same way you would if you were patching with compound, don't push the kife as you will gouge the walls with it. When I've finished a wall, I spray the next one and instead of taking 5, I wash the remaining paste off the previous wall with a clamshell sponge and clean water. Change the wash water often. When your finished give the walls another washing to get any leftover residue.
If the paste doesn't want to disolve there are a number other things to use instead of detergent. Club Soda is usually the first one I try, then I move to vinegar, the next (Your gonna love this one!), Beer. Yes, Beer. We jokingly reffer to this as a beer washing party. Buy a 12 pack, empty one can for every gallon of water in your sprayer, and drink the rest while you wash. If it doesn't remove the paste, it probably isn't going to come off, and after the rest of the beer, you won't care anyway!