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Contact paper? Not a good choice for walls. The adhesive on contact paper is very strong and very difficult to remove. Because the adhesive is also water-resistant, standard wallpaper removal products will not have any effect.
Instead, use a hair dryer to heat the contact paper. The heat will soften the adhesive and make removal much easier. Start at a corner and pull on the paper as you warm it. The warmth may also restore a little of the flexibility to the plastic so it is less likely to split as you remove it. Be patient and take your time or you may do incredible damage to the walls... especially if they are paper-faced wallboard!
Once the contact paper is off, you can use an adhesive remover on the walls to strip the adhesive residue, if any. Use a "citrus"-type adhesive remover rather than a solvent-based one since you may soften or even remove the paint with the solvent. Prime the walls before repainting to assure proper finish paint adhesion.