What can I do to get custody and visitation rights

I have been with my kids mother for 15 years. We have 4 kids total, 3 are my biological. We got married after the first 4 years into the relationship and divorced after 9 years of marriage. When we divorced in 2007, we had a planned visitation schedule but didn't require going through child support enforcement. Everything was okay, I even started paying her $400 every 2 weeks along with having my biological kids on my health insurance policy and still bought the kids needed accessories. In 2009, she decides to take me through child support in hopes of getting more than $400 that she was already getting every 2 weeks.

The adjusted support amount was decreased to $332 every 2 weeks. Since then she has kept the kids from me, not following the guidelines of our custodial rights that were agreed upon in the divorce. Just recently she cut all communication off with my kids including cell phone use, social networking, email and communication through the family. Everytime I came around to try and talk to my kids she threatened me by stating she had a restraining order against me, which later I found out was a false statement.

Now my question is, is there any rights as a paying non-custodial parent that I can exercise to get visitation rights to see and be with my kids? this has got to be wrong because I didn't bring my kids up just for them to become a bill statement in my life!