scrollviewer and treeview - get rid of the little rectangle in the bottom right corner

hey,

i wanna take out the little rectangle at he bottom right Corner of the scrollviewer of a treeview. But if i create a style for the scrollviewer it wont work, means, if i start my complete scrollviewer template with a style with the targettype scrollviewer, i won't applied, but if i start it with a style with the targettype scrollbar, it works.

I try to use VisualTreeHelper to get the Instanz of the Scrollviewer, but the Count of VisualChidrens of a treeview seems to be Zero, so, is there anybody how knows a Little neat trick to get rid of this Little rectanlge???