If you really want them gone, a satisfactory solution might be to go to a local sign shop - the kind that does stickers, logos, car striping, etc. They have sheets of very thin vinyl material and might have a matte black that you can use to cover the decals that might be OK. I got some scraps of black from a local shop that I've used, in a smaller way, to cover things on a bike frame I had. It looked OK.