Currently there is no out of the box way to disable this expansion on drop. You have to create your custom drag drop operation but this will be a lot of work for a small requirement. However, you can use the DropExpandDelay property of the RadTreeView and set it to a big TimeSpan interval , for example 2 hours.
You can find this demonstrated in the attached solution.
this.tree2.DropExpandDelay = TimeSpan.FromMinutes(120);
All the best,
the Telerik team