No matter how you approach the answer, you've pretty much answered the question yourself but just don't know how to go about 'flattening' the wall.
If no one can help here, visit your local Wallpaper Retailer or Home Center and discuss this w/them. Depending on how much work had to be done, I'd either scrape off what I could and apply a skim coat or 2 of spackle. Or I'd rip the drywall off and re-hang new drywall (if that's what you have.) I simply don't feel comfortable telling you which way to go.
Perhaps the Stores can tell you more. My best to ya and hope this helps.