Okay, well, I have a problem.

I can't stop my three year old son from masturbating. He literally won't stop. And it's pretty obvious that's what he's doing, even though he's obviously too young to orgasm. He always tries to take off his clothes, and he'll just sit their touching his penis and making these little cooing sounds. If I can convince him to keep his clothes on, he'll just rub his genitals against a pillow or toy.

He used to do it wherever he felt like it. I told him that that wasn't acceptable behaviour. I'd tell him no, or I'd spank him (not hard.) Well, he's smart enough to have realized that I don't like it, so now he does it sneakliy - in his room when I am not present, for example. I'll come in and he will try to hide it but it's painfully obvious. I seem to be able to keep him from doing it in public but not from doing it in general. He refuses to listen to me and insists on doing it in his room.

I don't know what to do. I am running out of ideas. How do I get it across to him that this is not acceptable behaviour?

ETA: As advised, I wanted to add for those who are confused that this is a religious and a moral issue, not an "omg masturbation is teh ew" issue.