By default ButtonDropDown controls used on Ribbon will allow end-user to add them to Quick Access Toolbar (QAT) and remove them from QAT by using the customization context menu which is displayed when button is right-clicked. To disable the customization for the button use Ribbon.IsCustomizable attached property.

For example this is how XAML would look like to disable the customization for a button:

<devco:ButtonDropDown Header="Open" Ribbon.IsCustomizable="False">