radTreeListView - allow selection of items only under same rootItem

[Biya]

how can make the telerik radTreeListView only allow selection of items under the same rootItem/ParentItem?

 UK
  -london
      -london 1
      -london --
      -london 100
  -Newcastle
      -Newcastle 1
      -Newcastle 2
      -Newcastle 3
  -liverpool
USA
  -New york
  -califonia
China
  -one
  -two

From above sample menu of a radTreeListView, user should be allowed to select only

UK, USA or China. OR London, Newcastle, Liverpool, OR Newcastle 1, Newcastle 2, Newcastle 3 BUT NOT Newcastle 1, London 1, UK ALSO NOT UK, Newcastle, Newcastle 1.

User should only select things/items at the same hierarchy. Basically, they should not select a parent and a child, THEY either select from parents or children or sub-children.

This is the control am using : http://www.telerik.com/help/silverlight/radtreelistview-overview.html