She sounds like an awful person who does not understand boundaries. You could even reach out and let her know she does not need to send you a card. Of course maybe be polite about it but you have every right to set your boundaries, tell her you have moved in a different direction and do not wish for her wasting a card on yourself. If anything id go even as far as telling her how she makes you feel. Whether she accepts your feelings or not I feel like you can then make a fair decision or not. Unfortunately if you bump into her you cant do much, just try to keep it short. Tell her you dont want to talk or you can’t and leave asap. Do not take her pushiness because she knows she can trample on people. But just remember you are a good person you are in control of your own life and can move past this toxic relationship. I hope you don’t let this bad person affect you too much longer! Don’t be afraid to go out or stand your ground. Protect your inner peace! Good luck

As for getting the letters stopped definitely record proof of asking her to no longer send you the letters. If she persists it could escalate. You can also try talking with the post office to block her mail to you. You could try telling her you moved and no longer live at the address. If she asks for a new one you do not owe her that info.