I've had casinos mark the back of slot cards with the word "VOID" with a marker and I was pretty successful using alcohol. What I actually used in one case was one of those small alcohol swab packets. If there was any residual, like a dark cloud kind of mark, I used a plain white plastic eraser and it came of very easily. In fact, I've used the white eraser to get off glue from scotch tape and other things from cards with any damage as long as the card is a hard plastic credit card type of card.