Has this enhancement been added?
An enhancement for this has been raised for a future release. For the time being, if you are talking about only one study area, you can run Clip tool to clip the Thiessen polygons by the study area polygon. If you have multiple study areas in the same feature class, you would need to do this one study area at a time in a loop (iteration).
I have made with arcobjects if you haven't license arcinfo: https://github.com/nicogis/Voronoi
I intersect with envelope ( https://github.com/nicogis/Voronoi/blob/master/GeometryVoronoi.cs#L180 ) but you can change with your polygon (study area)