3M makes an adhesive remover/solvent that works great. Also a similar product called Acrysol works the same. These are available at any automotive paint store or at some common auto parts stores. This wont harm the paint either. One caveat---soak a rag in the solvent and place over the decal for several minutes then gently warm as mentioned with a HAIR DRYER (a heat gun may ignite the solvent..ask me how I know!!!). The decals will peel off and the solvent will completely remove the residue left from the adhesive. I have removed many a decal from race cars over the years (sponsors can change like the wind) and this worked best in my hands.
Also, Honda makes a spray cleaner/wax that works great to take out deep grime without harming graphics and the wax in the product produces a nice shine and makes cleaning easier next time.
my $0.02 Robin A.