How to prevent camera from going down?

I want to control the camera from going down in Z axis.
Just assume a flat surface and the camera is on top. The camera should not go beyond that flat surface. 

I can get the event from HelixViewport3D_CameraChanged() event whenever the camera position is getting changed. But I cannot prevent from going down. e.Handled = true; is not working.

Basically I want to prevent camera position's Z axis should not go into negative.