I'm sorry to hear about what happened. However, right now, I have to say that from a legal standpoint, there's not much you can do because it was just one letter after 3 years of no contact.
If the harassment continues, such as him calling, writing, emailing, texting, etc., especially after you make it clear that you want him to stop, you could pursue a harassment protection order against him - a protective order from the court that legally tells him to stop harassing you or potentially face criminal charges; you could even file a police report. The reason you cannot do these things now is as I said, it was one letter after no contact from him for years, so it doesn't yet rise to the legal definition of harassment, which is a substantial, ongoing, continuous pattern of harassment done to cause fear, abuse, intimidation, etc. For example, to get a harassment protection order in Massachusetts, a person you are trying to get the order against must commit 3 or more acts against you.
One thing you could try now is having a local attorney draft a "cease and desist" letter to your father, making it clear that you no longer wish to hear from in any way, shape or form and that any ongoing contact from him may result in you filing civil and/or criminal actions against him to the fullest extent of the law. You know your father best obviously and would know if such a letter would only "fan the flames" and make things worse, but sometimes, a strongly worded letter from a lawyer threatening potential legal action is enough to make people back off.
Please let me know if I can clarify anything or provide additional information.