How to remove the custom option from the filter dropdown in WinForms GridGroupingControl?

this.gridGroupingControl1.TableControlCurrentCellShowingDropDown += gridGroupingControl1_TableControlCurrentCellShowingDropDown;
this.gridGroupingControl1.FilterBarSelectedItemChanging += gridGroupingControl1_FilterBarSelectedItemChanging;
//Avoids the index conflict problems in the SelectedIndex.
void gridGroupingControl1_FilterBarSelectedItemChanging(object sender, FilterBarSelectedItemChangingEventArgs e)
{
    if(e.SelectedIndex > 0)
    {
       e.SelectedIndex += 1;
    }
}
void gridGroupingControl1_TableControlCurrentCellShowingDropDown(object sender, GridTableControlCurrentCellShowingDropDownEventArgs e)
{
    //Removes custom option from the filterbar when the EnableLegacyStyle is default.
    if(e.TableControl.CurrentCell.Renderer is GridTableFilterBarCellRenderer)
    {
       GridComboBoxCellRenderer renderer = e.TableControl.CurrentCell.Renderer as GridComboBoxCellRenderer;
       renderer.ListBoxPart.Items.RemoveAt(1);
    }
    //When the EnableLegacyStyle is set to false, the options of the filterbar executes improper functions.  
    //Use the GridTableFilterBarGridListCellRenderer for avoiding these types of problems.
    if(e.TableControl.CurrentCell.Renderer is GridTableFilterBarGridListCellRenderer)
    {
       GridTableFilterBarGridListCellRenderer  renderer = e.TableControl.CurrentCell.Renderer as GridTableFilterBarGridListCellRenderer;
       renderer.ListControlPart.Items.RemoveAt(1);
    }
}

Private Me.gridGroupingControl1.TableControlCurrentCellShowingDropDown += AddressOf gridGroupingControl1_TableControlCurrentCellShowingDropDown
Private Me.gridGroupingControl1.FilterBarSelectedItemChanging += AddressOf gridGroupingControl1_FilterBarSelectedItemChanging
'Avoids the index conflict problems in the SelectedIndex.
Private Sub gridGroupingControl1_FilterBarSelectedItemChanging(ByVal sender As Object, ByVal e As FilterBarSelectedItemChangingEventArgs)
  If e.SelectedIndex > 0 Then
    e.SelectedIndex += 1
  End If
End Sub
Private Sub gridGroupingControl1_TableControlCurrentCellShowingDropDown(ByVal sender As Object, ByVal e As GridTableControlCurrentCellShowingDropDownEventArgs)
  'Removes custom option from the filterbar when the EnableLegacyStyle is default.
  If TypeOf e.TableControl.CurrentCell.Renderer Is GridTableFilterBarCellRenderer  Then
    Dim renderer As GridComboBoxCellRenderer = TryCast(e.TableControl.CurrentCell.Renderer, GridComboBoxCellRenderer)
    renderer.ListBoxPart.Items.RemoveAt(1)
  End If
  'When the EnableLegacyStyle is set to false, the options of the filterbar executes improper functions.  
  'Uses the GridTableFilterBarGridListCellRenderer for avoiding these types of problems.
  If TypeOf e.TableControl.CurrentCell.Renderer Is GridTableFilterBarGridListCellRenderer Then
    Dim renderer As GridTableFilterBarGridListCellRenderer = TryCast(e.TableControl.CurrentCell.Renderer, GridTableFilterBarGridListCellRenderer)
    renderer.ListControlPart.Items.RemoveAt(1)
  End If
End Sub