Search the forum for "Hiding the User Interface" and "Opening Form" topics.
There is a way if you have an opening form defined to make its "Form_Open" or "Form_Load" routine execute some code that would hide the navigation pane, ribbons, and other facets of the Access graphic user interface. What you are describing is sometimes called a "Switchboard" or "Dispatcher" form, and you could search the forum for those terms as well.