What was the date of first delinquency?
In the state of Indiana, the SOL to bring suit for credit card debt is six years.
If you've been delinquent for 6 years, filing suit is useless to them.
They can, however, keep selling the debt and bugging the daylights out of you until the heat death of the universe. All you can do about that is what you've been doing - send a cease and desist letter.