Good for you for writing a DA letter. Now, maybe they will leave you alone. But, it isn't a guarantee you will never hear from them again. JWs are notorious for not paying attention to territory cards that have a "Do Not Call". They are so busy gossiping they might not notice it and then you will have two idiots on your stoop.
If you lose your cool and tell them off, all you will accomplish is to furnish them with a good laugh in the car group. You might want to hand them some ex-JW literature. You should keep a stack of it handy just in case. That would be a better tactic than getting angry.
They will certainly get a DA letter from me if I ever get a tithing bill (like Moshe said in one of his posts ). They can go take a flying leap. I'd tell them to stick it where the sun don't shine!