To reproduce:
1.Use the Self-Referencing Hierarchy help article to fill the grid with data.
2.Subscribe to the ChildViewExpanding event and use the following code snippet:
private void radGridView1_ChildViewExpanding(object sender, ChildViewExpandingEventArgs e)
{
    if (e.ParentRow.ChildRows != null && e.ParentRow.ChildRows.Count > 0)
    {
        e.ParentRow.ChildRows[0].Delete();
    }
}