[vtkusers] set an actor to be not pickable?

[Giff]

Is it possible?
I am using vtkPointPicker to pick some of its vertexes; as a vertex is
picked, a new actor is created, to display a sphere on that vertex. When I
continue picking, however, I often end up inside some of the spheres rather
than on my object. How could I disable the possibility of the spheres to be
picked?

Thanks a lot in advance.
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