How many custom icons are you going to give them? One possibility is, instead of tool in a toolbar, create macros and assign them to buttons. Then, use View/Full Screen to hide the default tool bars. Of course, there's no way to prevent the user from going and turning off Full Screen. Never mind.
This would only be nessisary when a specific *.xls file is opened and that file would have to contain the VBA code to hide the toolbars. I would really like to avoid the full screen option as it would remove my custom toolbar.
probably just the first line in the macro above is the one needed if you "unselect" all others at default....anyways, this is what I use to restore the tool bars after hiding them:
' notools Macro
' Macro recorded 5/7/2002 by Randy
Application.CommandBars("Standard").Visible = True
Application.CommandBars("Formatting").Visible = True
Application.CommandBars("Visual Basic").Visible = True