You can inherit the RichTextEditorRibbonBar which will give you access to the drop-down list. Then you can hide the undesired items:
class CustomRichTextEditorRibbonBar : RichTextEditorRibbonBar
foreach (var item in dropDownListFont.FontListView.Items)
item.Visible = false;
I hope this helps. Should you have any other questions do not hesitate to ask.
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