Very fine sandpaper- wet sand, #600 or #1000. Since you are dullcoating, you are looking for a flat finish anyway. Even those fine sandpapers will leave you with a matt finish, but you can sand the decal off with the removal of only a very thin layer of paint. With those grits you have to sand quite awhile to get through a whole coat of paint.
Problem is, the dull finish will lead to silvering if you apply new decals, so you'd have to go through the whole process of local gloss coat, apply decals, and then dullcoat again.