To reproduce use the following code:
public Form1()
{
    InitializeComponent();
   
    radChartView1.AreaType = Telerik.WinControls.UI.ChartAreaType.Pie;

    radChartView1.ShowLegend = true;
    series.ValueMember = "CategoryID";
    series.DisplayMember = "CategoryName";
    series.DataSource = this.nwindDataSet1.Categories;
    radChartView1.Series.Add(series);
}

private void radButton1_Click(object sender, EventArgs e)
{
    this.categoriesTableAdapter.Fill(this.nwindDataSet1.Categories);
}

private void Form1_Load(object sender, EventArgs e)
{
    // TODO: This line of code loads data into the 'nwindDataSet1.Categories' table. You can move, or remove it, as needed.
    this.categoriesTableAdapter.Fill(this.nwindDataSet1.Categories);
}

Workaround: 
- Clear the items manually:
    this.radChartView1.ChartElement.LegendElement.StackElement.Children.Clear();