In the hierarchical layout in JGraphX (and I am the author of it) it is very difficult to take into account vertices that cannot be moved in the layout. If those vertices are fixed in position that do not lend themselves to easily being allocated a rank, the algorithms used just will not work.
It is possible to ignore vertices in the layout, but that is very different thing to do compared to not making them movable.
So the short answer is that it's not possible to take the movable flag into account in this layout with the algorithms used. If there is a specific case for this functionality that will not benefit the general reader, I suggest raising an issue on the github project issue tracker, rather than an extended discussion here