How to politely tell someone to.... (question for athiests)?

Ambiguity leaves them confused. I like to say things like, "is that so?" or "Interesting...." without giving them too much of my attention. But if you are polite once and say thanks but no, and they persist, and you are polite again and say I said no thank you, I'm not interested, and they persist, I don't think you are obligated to be polite a third time.