[Gtk-sharp-list] When using Drag and Drop in a TreeView, how do I keep from dropping between rows?

[Matthew Pirocchi]

I'm testing a window that looks something like this:

http://i.imgur.com/3OaT6.png

Dragging a Tag to a Card links the Tag to the Card. So does dragging a
Card to a Tag.

It's meaningless to drop a tag between two cards, or a card between
two tags. I can ignore these outcomes in the Handle...DataReceived
function like this:

if (dropPos != TreeViewDropPosition.IntoOrAfter &&
    dropPos != TreeViewDropPosition.IntoOrBefore)
    return;

However, when dragging, the user still sees the option to insert:

http://i.imgur.com/UU01L.png

How do I prevent this from happening?