Removing stubborn wallpaper adhesive can be challenging, but it is possible with the right tools and techniques. One effective method is to mix hot water and vinegar in a spray bottle and apply it to the adhesive. Let it sit for a few minutes to allow the solution to penetrate, then use a scraper or putty knife to gently remove the adhesive. You may need to repeat the process several times for particularly stubborn adhesive. Another option is to use a commercial wallpaper adhesive remover or a solvent-based solution, but be sure to follow the manufacturer's instructions carefully and use protective gloves and eyewear.