There are cleaning products made specifically to be used for the removal of ink, but they won't be able to penetrate that rubber well enough to make a difference I don't think. It honestly doesn't look that bad/isn't crazy noticeable. Sand paper or a comparable abrasive product will probably be your most effective bet though if you are not content as is, just rub away the layers the ink penetrated.
Just color it in a little with sharpie so it dosent look like a price, hit it with something to dull it out and call it a day. The rubber is porous and like @Jompso said you wont be able to get it out.