@HelpPlz,
(1) Start by admitting to yourself that you were wrong in telling him that he "could never play again." 'Never' is an awfully long time and, after all, in a few years he will be too old for you to be able to enforce that edict.
(2)Now, next you have to figure out how to lift the embargo without appearing that you're backing off. Some of the suggestions made by other posters are excellent. Making restitution to whomever he stole from is fundamental and absolutely necessary.
(3) The next time he asks whether he might, after all, play with those cards again, I suggest you sit down with him and talk the situation over in some detail. I get the uncomfortable feeling that there is a lack of communication between you two. Ask if he has an explanation for this lapse in good judgement. Ask what
he would do if he were in your shoes. Draw him out. He might well come up with a solution that you haven't thought of yourself and that turns out to be acceptable to you both.
Best of luck.'