only lighting the front of an object

I have a spot light orbiting a sphere.
The problem is, the back of the sphere behind the light is being lit too. It happens on the demo posted here a while ago
Is there a way to stop this?

If you draw a small sphere to simulate the shape and position of the light source, is it where you thought it would be?

The light source is in the right place, and shinging in the correct direction; with a narrow beam, the front side of the sphere has a spot lit correctly, but it’s as though the beam is continuing through the sphere to light the far side as well.
After experimentation, it seems that I have to set the lights ambient colour to zero to stop it.