Elke, we can remove a menu item from the context menu of
SfRichTextBoxAdv by using the respective menu item's header name. For example,
If you want to remove the 'New Comment' menu item, which is highlighted
below, from the context menu, iterate through the context menu items. Check
each item's header name for a match with 'New Comment'. If a match is found,
remove the menu item from the context menu.

We have created a sample application to demonstrate the same as mentioned
above. Additionally, we have included the code to add new menu items to the
context menu of SfRichTextBoxAdv, which is provided below.
Attachment:
SfRichTextBoxAdv_ContextMenu_88f4e003.zip