Much like with anything the abuser does, unless they step into illegality, there is not much you can “make” them stop doing. The same goes for these letters. So if making them stop is not an option, then you best strategy is: making your parents understand, what is going on.
Often we prefer not to share our story in great detail. Because we feel ashamed, because we feel people would not believe us, because it is too painful… Sharing your story is really tough, but it is part of the process of owning your story. When you can share your story, you can explain to your parents why you need them to keep your location safe.
If your parents continue to suggest you reconcile, you may feel inclined to take the emotion out of things. Stating you are an adult and –even if they cannot understand why- you are totally in your right to keep your address away from your ex. It is a decision that is not up to them, and you do not wish to discuss it any further. If they bring it up again, shut the discussion down immediately by stating: we agreed not to talk about this anymore.
Not everyone will see your side of things. You may decide that some relationships are beyond saving. Realize that your recovery and protection from further abuse are more important than anything, and anyone who truly loves you will understand and respect this!