How to deal with wrongful collection?

Couple months ago my car got towed. I went to small claims and won – the tow yard actually had to pay me money and the Judge said I didn’t owe any money to the tow yard. Now I’m receiving mails from collection agency trying to get me to pay money for towing the car. I’m not responsible for any debt as I already won the lawsuit.

How do I proceed? Should I contact the debt collector? If so, how? By mail? What should I write?