You just have to keep saying "no" until he gets the picture. Try not to do or say anything which he could misinterpret. It'll take time but he'll get the message eventually.

__________________
"When I was 4 years old, they tried to test my IQ.
They showed me this picture of 3 oranges and a
pear. They asked me which one was different and
did not belong. They taught me different was
wrong."
- Ani DiFranco -