May I withhold visitation because my spouse will not pay child support?

No, you cannot. Your divorce decree probably contains a warning that you may not withhold visitation for nonpayment of child support. What you can do, and probably should do, is file a motion to enforce payment of the child support. Your ex is in contempt of court if he or she is failing to pay child support in the amount and way ordered. As a general rule, all courts will enforce child support orders, and have to power to put non-payers in jail, which almost always results in restarting the flow of money for the children.