Why am I still paying CSA?

I have had a life of hell with these people for 19 years.

It started when a girl i had a date with fell pregnant we split up and she named me as the father i was summoned to the CSA office and treated like a criminal not being able to defend myself and it has never been proven that i am the father of the child.

Since then i have been paying for him when i ask for updates they never come forward with it.

The alleged child is 19 now and i am still paying £128 a week and getting sick of this as by law he should be an adult at 16 i need help to find out the law in this.