Microsoft MVP A.D. Tejpal submitted the following code some time ago to hide
the navigation pane:

Sample code in form's module - Access 2007
(for hiding / un-hiding NavPane window)
'====================================
' Declarations section
' Comma separated list of form names
' already in hidden state (so that these
' do not get un-hidden along with others)
Private HiddenFormsList As String
'-------------------------------------------------------

Private Sub CmdHide_Click()
   ' Hide all open forms & reports so that with
   ' statement (A) focus stays on NavPane even
   ' when no object is on display (e.g. all NavPane
   ' groups are in collapsed state)
   P_HideAllOpenFormsReports

   ' Make NavPane the active window and hide it
   DoCmd.SelectObject acForm, , True               ' (A)
   DoCmd.RunCommand acCmdWindowHide

   ' Unhide all open forms and reports (other than
   ' forms deliberately kept hidden).
   ' Activate this form
   P_UnHideAllOpenFormsReports
End Sub