To reproduce: 
1. Create a UserControl.
2. Drag from the Toolbox a RichTextEditorRibbonBar and drop it onto the UserControl.
3. Use the following code:
Sub New()
    InitializeComponent()
    Me.RichTextEditorRibbonBar1.CloseButton = False
    Me.RichTextEditorRibbonBar1.MinimizeButton = False
    Me.RichTextEditorRibbonBar1.MaximizeButton = False
End Sub

You will notice that the header is cut off.

Workaround: in order to hide the system buttons, use the following code:

 Me.RichTextEditorRibbonBar1.RibbonBarElement.RibbonCaption.SystemButtons.Visibility = Telerik.WinControls.ElementVisibility.Hidden