"Flossing" works on foam-adhered badges, but it won't work well on a decal.
If a hairdryer doesn't do the trick, get some General Purpose Adhesive Cleaner #051135-08984 from an auto body supply shop -- it will dissolve the glue without harming the paint. It's a great solvent for working around the car, so you'll find other uses for it as well.
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