Go to your local hardware store, find the biggest brush they have (about 7 inches wide, 2 inches thick or so should suffice, costs about $20 to $30), take a stiff painter's scraper (the scraper is stiff, not the painter, as opposed to those el cheapo easy-flex ones...scrapers again), get a bucket filled with water...now that you have all you need, dip the brush into the water, let it drip for a few seconds, slap the wet brush at the residue and rub it in. (Would really be easier to show you how). First you soak the wall by "painting" it with the water, then you rub it, sort of by holding the brush sideways and broadside, so the bristles touch the wall with almost their full length. Rub, brush, mop.(dammit, I'm tempted to send you a drawing how to hold the brush) Anyway, after fumbling around thus for while, you may scrape the stuff off. Repeat as needed, ask me if anything is unclear.