Hi Hanish,
Thank you for contacting Syncfusion support.
We have defined the context menu for the RichTextBoxAdv control in the control template. You can customize the control template based on your requirements. We have prepared a simple Silverlight 5 sample to demonstrate the same by editing a copy of the default control template to remove the options from default context menu of RichTextBoxAdv control.
Sample link:ContextMenu.zip
Please let us know if you have any questions.
Regards,
Dilli babu.