Car Registration Issue?

I sold my car that was registered in Florida. The new owner never registered the car in her name and she got a ticket in NY. Now the NY Port Authority is sending me nasty grams saying I need to pay the fine (no dang way am I paying it) How can I get this person to register the car or have Florida get it out of my name... Or have a lien put on it or have it repoed or something to force her hand.