If you want to just uncheck the item you can get the ACMenuItem object and set the Checked property to false.
If you don't want a complete menu items group to be checkable you can disable the complete group:
NavDrawer.NavigationView.Menu.SetGroupCheckable(2, False, False)
If you want a single item to be not checkable just disable the Checkable property:
NavDrawer.NavigationView.Menu.AddWithGroup2(2, 12, 1200, "Help", xml.GetDrawable("ic_help_black_24dp")).Checkable = False