Cancel a Drag & Drop for some specific items in a Gtk.TreeView

[<c buhtz posteo jp>]

I also opened this question on
https://stackoverflow.com/q/51974845/4865723

I have a Gtk.TreeView here. Most but not all of the items should be
able to be dragged & dropped. For example the first item should not be
able to be dragged & dropped but it should be selectable.

How can I realize this? Maybe I have to use the drag-begin signal and
stop the drag in there. But I don't know how.