I would just say that I have decided that therapy is not the thing to focus on at this time. I would not discuss it with the therapist at all if I had already decided it was not useful. They are not supposed to talk you into staying if you don't want to be there. Discussing it with a therapist can lead to at least two difficulties-sometimes people get hurt when the therapist does not try to convince them to stay OR the therapist talks someone into staying because the person feels guilty about leaving the therapist who seems to want them.
I would just tell the therapist I won't be back and that I will call about another appointment if I change my mind.