I am sorry to hear it. It’s so infuriating

Throw it in a trash. That would be my course of action

Now if you personally knew that person and she (or he) deliberately sent letters to your husband knowing that he is dead and she (or he) wanted you to suffer, then yes, I’d say tell them off. But she of he were likely just given list of names and they followed their particular congregation’s routine about sending letters and advertising their religion. I’d not contact them. More so if you contact them, they might be prompted to open further communication “now when you are in pain it’s even more crucial that you follow our faith etc”

Pitch it. Forget it.

Sorry again. Hang in there. Sending hugs