The trick is to spray an area only large enough for you to scrape off before it thoroughly soaks the drywall. Either way, you will probably have to do some repair of the drywall when your finished.
This is a messy project, so take the time to remove the furniture and use lots of plastic and dropcloths. Keep in mind that plastic on the floor during a wet process like this can be a slip hazzard. I use only drop cloths on the floor and protect the woodwork and windows with plastic.