Thank you for writing.
This option is actually available during design time. You can select Edit UI Elements from the smart tag of RadTreeView. There you will see the TreeViewElement on the left and you will be able to set every property in the Property Grid on the right. The property you should use to show/hide treeview's border is RadTreeViewElement.DrawBorder. You can also access this property runtime in the code (runtime) of your applicaiton.
Please write back if you need further assistance.
All the best,
the Telerik team