<telerik:RadTreeView  x:Name="radTreeView"
                             IsExpandOnSingleClickEnabled="True"
                             IsLoadOnDemandEnabled="true"
                             LoadOnDemand="RadTreeView_LoadOnDemand"
                            ItemPrepared="radTreeView_ItemPrepared"
                            ItemsSource="{Binding TreeViewSource,Mode=OneWay}"
                         ItemTemplate="{StaticResource ParentTemplate}"
                         />

private void radTreeView_ItemPrepared(object sender, RadTreeViewItemPreparedEventArgs e)
    {
        // get a reference to the item that has been selected
        RadTreeViewItem preparedItem = e.PreparedItem as RadTreeViewItem;
            preparedItem.IsLoadOnDemandEnabled = false;
    }