I had children with a man I thought was upright, come to find out he is a liar, cheater and worst of all a registered sex offender. I have all legal rights/custody to our children, but he does see them under my supervision. Can I, through the court system, request that he never be allowed to see them again, supervised or not?

You can request it but its not likely to happen. They could move it to supervised through the courts so its not in your home. What was he convicted of and did he incur a conviction that involved your children?

No it has nothing to do with our children and it is something to do with his past like 20 years ago.

You can always try, but it will not happen, he will get at least supervised, and perhaps even unsupervised after so many months of review by the courts.

Esp if this had nothing to do with children, and he has not proven hisself a threat to the kids.

In fact often if you try too hard to stop him without any real good cause, it will back fire in court against you.