Only a court of competent jurisdiction can vacate the child support order. If you fail to pay under the existing order you could be setting yourself up for a Contempt charge. If your ex no longer wishes to receive payments from you for the children, she will have to go to the Probate & Family court that issued the order and seek a new order terminating the child support obligations. She will have to give GOOD CAUSE why the court should absolve you totally of your support obligations. Unless she just won the PowerBall lottery and/or is independently wealthy, she will be hard pressed to explain why you should be exempted from the absolute duty to support your children. Another reason why the court might terminate your obligation is in the case of domestic violence. In the past, where a woman or the children were in danger of retribution from paying spouse, she could claim good cause due to danger of physical harm if the paying spouse knew their whereabouts. Today, this is less likely because the court can arrange to have the payment made to the Department of Revenue for benefit of the children and the spouse need never have any contact with the wife or children. This will depend on state laws.
Until such time that a court has terminated your child support obligation, I strongly advise you to continue making your payments. If she refuses to accept payments or fails to negotiate your checks, place the money in a trust or escrow account for the benefit of the children. If the contempt charges should later surface, you will have a very good defense and you will have the money to make the arrears payments.
The bottom line is: DO NOT STOP YOUR PAYMENTS UNTIL A COURT ORDER TERMINATES OR MODIFIES YOUR OBLIGATION. If you wife should actually take this matter before a judge, then you should retain counsel to assist you navigate this tricky situation.