Child Support and Visitation

I have 3 children with my ex-wife and pay monthly child support directly to her as part of the separation agreement. My understanding is that this a contract. My oldest daughter turned 18 in September 2009, at which point I discontinued child support for her in the amount of 1/3 less. I still pay support for my other children. My ex-wife contends that since she still carries her on her NC State Employees health insurance plan that I should still pay support. Her insurance is the same for 3 children as it is for 2, or 1. Should I still be paying support for my oldest daughter? Have I been breaking a contract?

Second, the divorce agreement provides for reasonable visitation rights and certain holidays. My 2 youngest children participate in high school activities that preclude visitation, which I agree with; however, they have certain weekends free. My ex-wife is currently denying my, and their, visitation rights as punishment for them being disrespectful to her AND not helping enough around the house.

What is your opinion.

It depends on what your Agreement says with respect to the termination of child support for any one child.
You may sue her for breach of contract relating to the visitation.

There is no language included regarding termination. What would the precedent be in this case?

The statutes describe various factors for termination of support;
1.) If the child is emancipated;
2.) If the child is still in primary or secondary school when the child reaches 18 support payments continue until the child graduates or otherwise ceases to attend school on a regular basis, fails to make satisfactory academic progress or reaches the age of 20, whichever occurs first, unless the court, in its discretion orders that support cease at age 18 or prior to graduation from high school.