First, your child must be pretty honest with you to admit that he/she is playing these games. If that's the case, acknowledge that honesty and don't punish the infraction or your child will learn that it may be better to lie to you than to admit something that might result in punishment. You can go over the reasons why you don't like these games and hope for the best. You should let the other parent know that you don't want your child to play these games and hope that he or she will respect your wishes. Finally, remember that you can't always be around your kid and sometimes you have to trust that they've taken the knowledge you've given them and will find their own way by trying out different things and coming back (we hope) to the values you hold dear.