sometimes scraping with a sharp wide stiff scraper will work, and sometimes its so hard nothing will work. Sanding is out of the question, the mess is too much and you will spend alot of money on paper.
I would scrape and be very careful not to gouge the wall board or remove the wall board skin.
Sometimes after hard work and no progression, one might consider just attaching new 1/4 wall board over the old and adjusting molding and trim for the addition. This is a fresh start and not too difficult to accomplish. You will need to mud and tape and prime. Let this be your last alternative.