I am not sure what your rights are in your corresponding state. I think I would research what your rights are in your state as a birth mother. If your biological child has found you and you do not want a reunion, I will suggest telling your child this, as kindly as you can and the reason why. From the child's perspective, even an adult child, it will feel like being abandoned, again.
If you can give your child a reason to why you do not want to go forth with the reunion, it will help the adult child not internalize that something is wrong with them.
Maybe in time you might want a connection with this child.