The RadMultiColumnComboBox has a separate keyboard functionality executed when the watermark textbox (the search box) is focused and the dropdown is open. Basically, you can navigate through the rows of the gridview part and select them using the keyboard.
Currently, this functionality is private. Introduce an API that allows you to customize the keyboard navigation. Consider something similar to the KeyboardCommandProvider of RadGridView.
Add non-editable feature to MultiColumnComboBox ( like on ComboBox ).