How to find points lie in unbounded voronoi cells?

Let P be the set of n points and Q is a subset of P of cardinality k. I want to make (at most) k clusters of P based on Voronoi diagram of Q. I could find the points lie in the bounded Voronoi cells using the routine 'inpolygon'. How do I find the points of P lie in the unbounded cells? I thought of having a bounding box of the Vornoi Diagram, but I do not know how to implement it. Please help me or provide me some source.

Thanks in advance