Set a fixed control point in vtk contour widget
===

I am doing a contour widget in VTK according to the example below:

http://www.vtk.org/Wiki/VTK/Examples/Cxx/Widgets/ContourWidget

I want to make a certain control point as fixed, (first and last nodes) so the user can't drag them.
I tried many methods, such as keypressactivationoff, draggableoff etc but all of them were only for the whole contour widget or contour representation. 
How can I do it only for a given point?
Is there any way to do this?
I added the question for the vtk mailing list is as well:

http://comments.gmane.org/gmane.comp.lib.vtk.user/64400

Please help me.

Thanks.