I'm thinking of telling my dad to stopping sending me cards and letters.
Why do you want your dad to stop sending you cards and letters?
I don't want to have any communication with him anymore.
He probably thinks he's doing a nice thing by sending them to you. Regardless, let him know you don't appreciate them and that you don't want to communicate with him any more.
I have to tell him how I feel, it's the right thing to do.
It's important to be honest with your parents and tell them why you don't want to communicate with them anymore. Your father needs to know where your relationship currently stands and how you feel.