I want to create a clean user interface for my students to use QGIS. I tried to remove the menu bar using the customization in setting but I am not able to remove everything, e.g. Database, plugin, etc. Is there a way to remove them?
If you go the the Settings > Customisation menu options, it gives you a dialog that allows you to turn off whole toolbars and also remove menus and docks. You can also remove individual buttons or menu items too rather than the whole toolbar or menu.
You can also save these settings from this panel to an ini file, and then open an ini file on another machine.
This blog post talks about the feature:
Vector, Database and Web menu entries should vanish if you deactivate all connected plugins. For vector that would be (at least) ftools plugin.